diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 8ae32ca..a4cc69f 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -3,6 +3,9 @@
 ## Unreleased
 
 ### Changed
+- Renamed the internal language state from help-oriented names to
+  UI-language names (`ui_lang_t`, `client->ui_lang`, and
+  `i18n_*_ui_lang`) so future i18n work has a correctly named seam.
 - Command names, aliases, help summaries, concise-manual command rows, and
   unknown-command suggestions now share a dedicated `command_catalog` module.
 - COMMAND-mode output is now a small scrollable pager with `j/k`, page
@@ -28,7 +31,7 @@
 ### Added
 - Added a first i18n boundary: `TNT_LANG` / locale detection now chooses the
   default interactive UI language (`en` or `zh`) for username prompts, status
-  hints, help language, and `:support`.
+  hints, help output, and `:support`.
 - Added `:lang <en|zh>` so users can switch the interactive UI language for
   their current session.
 - COMMAND-mode `:help`, unknown-command guidance, language command output, and
diff --git a/docs/Development-Guide.md b/docs/Development-Guide.md
index 1b091d5..6ffd6a8 100644
--- a/docs/Development-Guide.md
+++ b/docs/Development-Guide.md
@@ -417,6 +417,9 @@ keys.
 
 2. **Stable language identifiers**
    - Interactive `:lang` accepts only stable language codes: `en` and `zh`.
+   - Code should name this concept `ui_lang`, not `help_lang`; the same value
+     controls prompts, status text, help, command output, MOTD chrome, and
+     system messages.
    - Locale detection may accept locale-shaped values such as
      `en_US.UTF-8`, `zh_CN.UTF-8`, `C`, and `POSIX`.
    - Do not accept natural-language labels such as `english`, `chinese`,
diff --git a/include/cli_text.h b/include/cli_text.h
index ae08dc5..5fb6420 100644
--- a/include/cli_text.h
+++ b/include/cli_text.h
@@ -4,9 +4,9 @@
 #include "common.h"
 
 void cli_text_append_help(char *buffer, size_t buf_size, size_t *pos,
-                          const char *program_name, help_lang_t lang);
-const char *cli_text_invalid_port_format(help_lang_t lang);
-const char *cli_text_unknown_option_format(help_lang_t lang);
-const char *cli_text_short_usage_format(help_lang_t lang);
+                          const char *program_name, ui_lang_t lang);
+const char *cli_text_invalid_port_format(ui_lang_t lang);
+const char *cli_text_unknown_option_format(ui_lang_t lang);
+const char *cli_text_short_usage_format(ui_lang_t lang);
 
 #endif /* CLI_TEXT_H */
diff --git a/include/command_catalog.h b/include/command_catalog.h
index 900a086..891078c 100644
--- a/include/command_catalog.h
+++ b/include/command_catalog.h
@@ -30,8 +30,8 @@ bool command_catalog_match(const char *line, tnt_command_id_t *id,
                            const char **args);
 const char *command_catalog_suggest(const char *name);
 void command_catalog_append_full(char *buffer, size_t buf_size, size_t *pos,
-                                 help_lang_t lang);
+                                 ui_lang_t lang);
 void command_catalog_append_manual(char *buffer, size_t buf_size, size_t *pos,
-                                   help_lang_t lang);
+                                   ui_lang_t lang);
 
 #endif /* COMMAND_CATALOG_H */
diff --git a/include/common.h b/include/common.h
index 73edf40..5c818d2 100644
--- a/include/common.h
+++ b/include/common.h
@@ -44,11 +44,11 @@ typedef enum {
     MODE_HELP
 } client_mode_t;
 
-/* Help language */
+/* UI language */
 typedef enum {
-    LANG_EN,
-    LANG_ZH
-} help_lang_t;
+    UI_LANG_EN,
+    UI_LANG_ZH
+} ui_lang_t;
 
 /* Runtime helpers */
 const char* tnt_state_dir(void);
diff --git a/include/help_text.h b/include/help_text.h
index 8441e31..d2ff141 100644
--- a/include/help_text.h
+++ b/include/help_text.h
@@ -4,6 +4,6 @@
 #include "common.h"
 
 void help_text_append_full(char *buffer, size_t buf_size, size_t *pos,
-                           help_lang_t lang);
+                           ui_lang_t lang);
 
 #endif /* HELP_TEXT_H */
diff --git a/include/i18n.h b/include/i18n.h
index 37e9134..c2c5989 100644
--- a/include/i18n.h
+++ b/include/i18n.h
@@ -63,10 +63,10 @@ typedef enum {
     I18N_EXEC_UNKNOWN_COMMAND_FORMAT
 } i18n_text_id_t;
 
-bool i18n_try_parse_lang(const char *value, help_lang_t *lang);
-help_lang_t i18n_parse_lang(const char *value, help_lang_t fallback);
-help_lang_t i18n_default_lang(void);
-const char *i18n_lang_code(help_lang_t lang);
-const char *i18n_text(help_lang_t lang, i18n_text_id_t id);
+bool i18n_try_parse_ui_lang(const char *value, ui_lang_t *lang);
+ui_lang_t i18n_parse_ui_lang(const char *value, ui_lang_t fallback);
+ui_lang_t i18n_default_ui_lang(void);
+const char *i18n_ui_lang_code(ui_lang_t lang);
+const char *i18n_text(ui_lang_t lang, i18n_text_id_t id);
 
 #endif /* I18N_H */
diff --git a/include/manual.h b/include/manual.h
index ea52944..530d37e 100644
--- a/include/manual.h
+++ b/include/manual.h
@@ -4,6 +4,6 @@
 #include "common.h"
 
 void manual_append_interactive_panel(char *buffer, size_t buf_size,
-                                     size_t *pos, help_lang_t lang);
+                                     size_t *pos, ui_lang_t lang);
 
 #endif /* MANUAL_H */
diff --git a/include/manual_text.h b/include/manual_text.h
index 425e0ee..50969f4 100644
--- a/include/manual_text.h
+++ b/include/manual_text.h
@@ -4,6 +4,6 @@
 #include "common.h"
 
 void manual_text_append_interactive(char *buffer, size_t buf_size,
-                                    size_t *pos, help_lang_t lang);
+                                    size_t *pos, ui_lang_t lang);
 
 #endif /* MANUAL_TEXT_H */
diff --git a/include/ssh_server.h b/include/ssh_server.h
index 7dfc116..cab8fa1 100644
--- a/include/ssh_server.h
+++ b/include/ssh_server.h
@@ -26,7 +26,7 @@ typedef struct client {
     _Atomic int width;
     _Atomic int height;
     client_mode_t mode;
-    help_lang_t help_lang;
+    ui_lang_t ui_lang;
     int scroll_pos;
     bool follow_tail;                /* NORMAL stays pinned to latest until user scrolls up */
     int help_scroll_pos;
diff --git a/include/system_message.h b/include/system_message.h
index a82ece7..27c1dc9 100644
--- a/include/system_message.h
+++ b/include/system_message.h
@@ -5,11 +5,11 @@
 #include "message.h"
 
 void system_message_make_join(message_t *msg, const char *username,
-                              help_lang_t lang);
+                              ui_lang_t lang);
 void system_message_make_leave(message_t *msg, const char *username,
-                               help_lang_t lang);
+                               ui_lang_t lang);
 void system_message_make_nick(message_t *msg, const char *old_name,
-                              const char *new_name, help_lang_t lang);
+                              const char *new_name, ui_lang_t lang);
 
 bool system_message_is_system(const message_t *msg);
 bool system_message_is_join_leave(const message_t *msg);
diff --git a/src/cli_text.c b/src/cli_text.c
index a9ff7ca..87f8249 100644
--- a/src/cli_text.c
+++ b/src/cli_text.c
@@ -1,12 +1,12 @@
 #include "cli_text.h"
 
 void cli_text_append_help(char *buffer, size_t buf_size, size_t *pos,
-                          const char *program_name, help_lang_t lang) {
+                          const char *program_name, ui_lang_t lang) {
     const char *program = (program_name && program_name[0] != '\0')
                               ? program_name
                               : "tnt";
 
-    if (lang == LANG_ZH) {
+    if (lang == UI_LANG_ZH) {
         buffer_appendf(buffer, buf_size, pos,
                        "tnt %s - 匿名 SSH 聊天服务器\n\n"
                        "用法: %s [选项]\n\n"
@@ -48,15 +48,15 @@ void cli_text_append_help(char *buffer, size_t buf_size, size_t *pos,
                    TNT_VERSION, program, DEFAULT_PORT);
 }
 
-const char *cli_text_invalid_port_format(help_lang_t lang) {
-    return lang == LANG_ZH ? "端口无效: %s\n" : "Invalid port: %s\n";
+const char *cli_text_invalid_port_format(ui_lang_t lang) {
+    return lang == UI_LANG_ZH ? "端口无效: %s\n" : "Invalid port: %s\n";
 }
 
-const char *cli_text_unknown_option_format(help_lang_t lang) {
-    return lang == LANG_ZH ? "未知选项: %s\n" : "Unknown option: %s\n";
+const char *cli_text_unknown_option_format(ui_lang_t lang) {
+    return lang == UI_LANG_ZH ? "未知选项: %s\n" : "Unknown option: %s\n";
 }
 
-const char *cli_text_short_usage_format(help_lang_t lang) {
-    return lang == LANG_ZH ? "用法: %s [-p PORT] [-d DIR] [-h]\n"
+const char *cli_text_short_usage_format(ui_lang_t lang) {
+    return lang == UI_LANG_ZH ? "用法: %s [-p PORT] [-d DIR] [-h]\n"
                            : "Usage: %s [-p PORT] [-d DIR] [-h]\n";
 }
diff --git a/src/command_catalog.c b/src/command_catalog.c
index 6575c78..8d4cef9 100644
--- a/src/command_catalog.c
+++ b/src/command_catalog.c
@@ -221,11 +221,11 @@ const char *command_catalog_suggest(const char *name) {
 }
 
 void command_catalog_append_full(char *buffer, size_t buf_size, size_t *pos,
-                                 help_lang_t lang) {
+                                 ui_lang_t lang) {
     for (size_t i = 0; i < sizeof(entries) / sizeof(entries[0]); i++) {
-        const char *usage = lang == LANG_ZH ? entries[i].full_usage_zh
+        const char *usage = lang == UI_LANG_ZH ? entries[i].full_usage_zh
                                             : entries[i].full_usage_en;
-        const char *summary = lang == LANG_ZH ? entries[i].summary_zh
+        const char *summary = lang == UI_LANG_ZH ? entries[i].summary_zh
                                               : entries[i].summary_en;
         buffer_appendf(buffer, buf_size, pos, "  %-40s - %s\n",
                        usage, summary);
@@ -233,7 +233,7 @@ void command_catalog_append_full(char *buffer, size_t buf_size, size_t *pos,
 }
 
 void command_catalog_append_manual(char *buffer, size_t buf_size, size_t *pos,
-                                   help_lang_t lang) {
+                                   ui_lang_t lang) {
     for (int group = 1; group <= 3; group++) {
         bool first = true;
 
@@ -243,7 +243,7 @@ void command_catalog_append_manual(char *buffer, size_t buf_size, size_t *pos,
             if (entries[i].manual_group != group) {
                 continue;
             }
-            usage = lang == LANG_ZH ? entries[i].manual_usage_zh
+            usage = lang == UI_LANG_ZH ? entries[i].manual_usage_zh
                                     : entries[i].manual_usage_en;
             if (!usage) {
                 continue;
diff --git a/src/commands.c b/src/commands.c
index 502d279..746f3ad 100644
--- a/src/commands.c
+++ b/src/commands.c
@@ -93,17 +93,17 @@ void commands_dispatch(client_t *client) {
     if (!command_catalog_match(cmd, &command_id, &arg)) {
         const char *suggestion = command_catalog_suggest(cmd);
         buffer_appendf(output, sizeof(output), &pos,
-                       i18n_text(client->help_lang,
+                       i18n_text(client->ui_lang,
                                  I18N_UNKNOWN_COMMAND_FORMAT),
                        cmd);
         if (suggestion) {
             buffer_appendf(output, sizeof(output), &pos,
-                           i18n_text(client->help_lang,
+                           i18n_text(client->ui_lang,
                                      I18N_DID_YOU_MEAN_FORMAT),
                            suggestion);
         }
         buffer_appendf(output, sizeof(output), &pos, "%s",
-                       i18n_text(client->help_lang, I18N_UNKNOWN_GUIDANCE));
+                       i18n_text(client->ui_lang, I18N_UNKNOWN_GUIDANCE));
         goto cmd_done;
     }
 
@@ -112,7 +112,7 @@ void commands_dispatch(client_t *client) {
         int total = g_room->client_count;
         buffer_appendf(output, sizeof(output), &pos,
                        "\033[1;36m%s\033[0m  \033[2;37m· %d\033[0m\n",
-                       i18n_text(client->help_lang, I18N_USERS_TITLE), total);
+                       i18n_text(client->ui_lang, I18N_USERS_TITLE), total);
 
         time_t now = time(NULL);
         for (int i = 0; i < total; i++) {
@@ -137,25 +137,25 @@ void commands_dispatch(client_t *client) {
 
     } else if (command_id == TNT_COMMAND_HELP) {
         manual_append_interactive_panel(output, sizeof(output), &pos,
-                                        client->help_lang);
+                                        client->ui_lang);
 
     } else if (command_id == TNT_COMMAND_LANG) {
-        help_lang_t next_lang;
+        ui_lang_t next_lang;
 
         if (!arg || arg[0] == '\0') {
             buffer_appendf(output, sizeof(output), &pos,
-                           i18n_text(client->help_lang,
+                           i18n_text(client->ui_lang,
                                      I18N_LANG_CURRENT_FORMAT),
-                           i18n_lang_code(client->help_lang));
-        } else if (i18n_try_parse_lang(arg, &next_lang)) {
-            client->help_lang = next_lang;
+                           i18n_ui_lang_code(client->ui_lang));
+        } else if (i18n_try_parse_ui_lang(arg, &next_lang)) {
+            client->ui_lang = next_lang;
             buffer_appendf(output, sizeof(output), &pos,
-                           i18n_text(client->help_lang,
+                           i18n_text(client->ui_lang,
                                      I18N_LANG_SET_FORMAT),
-                           i18n_lang_code(client->help_lang));
+                           i18n_ui_lang_code(client->ui_lang));
         } else {
             buffer_appendf(output, sizeof(output), &pos,
-                           i18n_text(client->help_lang,
+                           i18n_text(client->ui_lang,
                                      I18N_LANG_UNSUPPORTED_FORMAT),
                            arg);
         }
@@ -172,7 +172,7 @@ void commands_dispatch(client_t *client) {
 
         if (target_name[0] == '\0' || rest[0] == '\0') {
             buffer_appendf(output, sizeof(output), &pos, "%s",
-                           i18n_text(client->help_lang, I18N_MSG_USAGE));
+                           i18n_text(client->ui_lang, I18N_MSG_USAGE));
         } else {
             bool found = false;
             client_t *target = NULL;
@@ -220,12 +220,12 @@ void commands_dispatch(client_t *client) {
 
             if (found) {
                 buffer_appendf(output, sizeof(output), &pos,
-                               i18n_text(client->help_lang,
+                               i18n_text(client->ui_lang,
                                          I18N_MSG_SENT_FORMAT),
                                target_name);
             } else {
                 buffer_appendf(output, sizeof(output), &pos,
-                               i18n_text(client->help_lang,
+                               i18n_text(client->ui_lang,
                                          I18N_MSG_USER_NOT_FOUND_FORMAT),
                                target_name);
             }
@@ -245,12 +245,12 @@ void commands_dispatch(client_t *client) {
 
         buffer_appendf(output, sizeof(output), &pos,
                        "\033[1;36m%s\033[0m  \033[2;37m· %d\033[0m\n",
-                       i18n_text(client->help_lang, I18N_INBOX_TITLE),
+                       i18n_text(client->ui_lang, I18N_INBOX_TITLE),
                        snap_count);
         if (snap_count == 0) {
             buffer_appendf(output, sizeof(output), &pos,
                            "  \033[2;37m%s\033[0m\n",
-                           i18n_text(client->help_lang, I18N_INBOX_EMPTY));
+                           i18n_text(client->ui_lang, I18N_INBOX_EMPTY));
         }
         for (int i = 0; i < snap_count; i++) {
             char ts[20];
@@ -268,10 +268,10 @@ void commands_dispatch(client_t *client) {
 
         if (new_name[0] == '\0') {
             buffer_appendf(output, sizeof(output), &pos, "%s",
-                           i18n_text(client->help_lang, I18N_NICK_USAGE));
+                           i18n_text(client->ui_lang, I18N_NICK_USAGE));
         } else if (!is_valid_username(new_name)) {
             buffer_appendf(output, sizeof(output), &pos, "%s",
-                           i18n_text(client->help_lang, I18N_NICK_INVALID));
+                           i18n_text(client->ui_lang, I18N_NICK_INVALID));
         } else {
             char validated_name[MAX_USERNAME_LEN];
             snprintf(validated_name, sizeof(validated_name), "%s", new_name);
@@ -303,22 +303,22 @@ void commands_dispatch(client_t *client) {
 
             if (taken) {
                 buffer_appendf(output, sizeof(output), &pos,
-                               i18n_text(client->help_lang,
+                               i18n_text(client->ui_lang,
                                          I18N_NICK_TAKEN_FORMAT),
                                validated_name);
             } else if (strcmp(validated_name, old_name) == 0) {
                 buffer_appendf(output, sizeof(output), &pos, "%s",
-                               i18n_text(client->help_lang,
+                               i18n_text(client->ui_lang,
                                          I18N_NICK_UNCHANGED));
             } else {
                 message_t nick_msg;
                 system_message_make_nick(&nick_msg, old_name,
-                                         client->username, client->help_lang);
+                                         client->username, client->ui_lang);
                 room_broadcast(g_room, &nick_msg);
                 message_save(&nick_msg);
 
                 buffer_appendf(output, sizeof(output), &pos,
-                               i18n_text(client->help_lang,
+                               i18n_text(client->ui_lang,
                                          I18N_NICK_CHANGED_FORMAT),
                                old_name, client->username);
             }
@@ -332,7 +332,7 @@ void commands_dispatch(client_t *client) {
             long val = strtol(arg, &endp, 10);
             if (*endp != '\0' || val < 1 || val > 50) {
                 buffer_appendf(output, sizeof(output), &pos, "%s",
-                               i18n_text(client->help_lang, I18N_LAST_USAGE));
+                               i18n_text(client->ui_lang, I18N_LAST_USAGE));
                 goto cmd_done;
             }
             n = (int)val;
@@ -341,7 +341,7 @@ void commands_dispatch(client_t *client) {
         message_t *last_msgs = NULL;
         int last_count = message_load(&last_msgs, n);
         buffer_appendf(output, sizeof(output), &pos,
-                       i18n_text(client->help_lang, I18N_LAST_HEADER_FORMAT),
+                       i18n_text(client->ui_lang, I18N_LAST_HEADER_FORMAT),
                        last_count);
         for (int i = 0; i < last_count; i++) {
             char ts[20];
@@ -358,12 +358,12 @@ void commands_dispatch(client_t *client) {
         while (*query == ' ') query++;
         if (*query == '\0') {
             buffer_appendf(output, sizeof(output), &pos, "%s",
-                           i18n_text(client->help_lang, I18N_SEARCH_USAGE));
+                           i18n_text(client->ui_lang, I18N_SEARCH_USAGE));
         } else {
             message_t *found = NULL;
             int found_count = message_search(query, &found, 15);
             buffer_appendf(output, sizeof(output), &pos,
-                           i18n_text(client->help_lang,
+                           i18n_text(client->ui_lang,
                                      I18N_SEARCH_HEADER_FORMAT),
                            query, found_count);
             for (int i = 0; i < found_count; i++) {
@@ -386,8 +386,8 @@ void commands_dispatch(client_t *client) {
     } else if (command_id == TNT_COMMAND_MUTE_JOINS) {
         client->mute_joins = !client->mute_joins;
         buffer_appendf(output, sizeof(output), &pos,
-                       i18n_text(client->help_lang, I18N_MUTE_JOINS_FORMAT),
-                       i18n_text(client->help_lang,
+                       i18n_text(client->ui_lang, I18N_MUTE_JOINS_FORMAT),
+                       i18n_text(client->ui_lang,
                                  client->mute_joins ?
                                  I18N_MUTE_JOINS_MUTED :
                                  I18N_MUTE_JOINS_UNMUTED));
@@ -398,7 +398,7 @@ void commands_dispatch(client_t *client) {
 
     } else if (command_id == TNT_COMMAND_CLEAR) {
         buffer_appendf(output, sizeof(output), &pos, "%s",
-                       i18n_text(client->help_lang, I18N_CLEAR_DONE));
+                       i18n_text(client->ui_lang, I18N_CLEAR_DONE));
     }
 
 cmd_done:
diff --git a/src/exec.c b/src/exec.c
index 8add1c4..9289de7 100644
--- a/src/exec.c
+++ b/src/exec.c
@@ -116,7 +116,7 @@ static void resolve_exec_username(const client_t *client, char *buffer,
 }
 
 static int exec_command_help(client_t *client) {
-    const char *help_text = i18n_text(client->help_lang, I18N_EXEC_HELP);
+    const char *help_text = i18n_text(client->ui_lang, I18N_EXEC_HELP);
 
     return client_send(client, help_text, strlen(help_text)) == 0 ? 0 : 1;
 }
@@ -285,7 +285,7 @@ static int exec_command_tail(client_t *client, const char *args) {
 
     if (parse_tail_count(args, &requested) < 0) {
         client_printf(client, "%s",
-                      i18n_text(client->help_lang, I18N_EXEC_TAIL_USAGE));
+                      i18n_text(client->ui_lang, I18N_EXEC_TAIL_USAGE));
         return 64;
     }
 
@@ -339,7 +339,7 @@ static int exec_command_post(client_t *client, const char *args) {
 
     if (!args || args[0] == '\0') {
         client_printf(client, "%s",
-                      i18n_text(client->help_lang, I18N_EXEC_POST_USAGE));
+                      i18n_text(client->ui_lang, I18N_EXEC_POST_USAGE));
         return 64;
     }
 
@@ -349,13 +349,13 @@ static int exec_command_post(client_t *client, const char *args) {
 
     if (content[0] == '\0') {
         client_printf(client, "%s",
-                      i18n_text(client->help_lang, I18N_EXEC_POST_EMPTY));
+                      i18n_text(client->ui_lang, I18N_EXEC_POST_EMPTY));
         return 64;
     }
 
     if (!utf8_is_valid_string(content)) {
         client_printf(client, "%s",
-                      i18n_text(client->help_lang,
+                      i18n_text(client->ui_lang,
                                 I18N_EXEC_POST_INVALID_UTF8));
         return 1;
     }
@@ -426,7 +426,7 @@ int exec_dispatch(client_t *client) {
     if (strcmp(cmd, "users") == 0) {
         if (args && strcmp(args, "--json") != 0) {
             client_printf(client, "%s",
-                          i18n_text(client->help_lang,
+                          i18n_text(client->ui_lang,
                                     I18N_EXEC_USERS_USAGE));
             return 64;
         }
@@ -435,7 +435,7 @@ int exec_dispatch(client_t *client) {
     if (strcmp(cmd, "stats") == 0) {
         if (args && strcmp(args, "--json") != 0) {
             client_printf(client, "%s",
-                          i18n_text(client->help_lang,
+                          i18n_text(client->ui_lang,
                                     I18N_EXEC_STATS_USAGE));
             return 64;
         }
@@ -452,7 +452,7 @@ int exec_dispatch(client_t *client) {
     }
 
     client_printf(client,
-                  i18n_text(client->help_lang,
+                  i18n_text(client->ui_lang,
                             I18N_EXEC_UNKNOWN_COMMAND_FORMAT),
                   cmd);
     return 64;
diff --git a/src/help_text.c b/src/help_text.c
index 96c630d..8248aea 100644
--- a/src/help_text.c
+++ b/src/help_text.c
@@ -3,8 +3,8 @@
 #include "command_catalog.h"
 
 void help_text_append_full(char *buffer, size_t buf_size, size_t *pos,
-                           help_lang_t lang) {
-    if (lang == LANG_EN) {
+                           ui_lang_t lang) {
+    if (lang == UI_LANG_EN) {
         buffer_appendf(buffer, buf_size, pos,
                        "TNT KEY REFERENCE\n"
                        "\n"
diff --git a/src/i18n.c b/src/i18n.c
index bc4fb29..f286eac 100644
--- a/src/i18n.c
+++ b/src/i18n.c
@@ -36,38 +36,38 @@ static bool starts_with_lang(const char *value, const char *prefix) {
     return is_lang_boundary(value);
 }
 
-bool i18n_try_parse_lang(const char *value, help_lang_t *lang) {
+bool i18n_try_parse_ui_lang(const char *value, ui_lang_t *lang) {
     if (!value || value[0] == '\0') {
         return false;
     }
 
     if (starts_with_lang(value, "zh")) {
-        if (lang) *lang = LANG_ZH;
+        if (lang) *lang = UI_LANG_ZH;
         return true;
     }
 
     if (starts_with_lang(value, "en") ||
         starts_with_lang(value, "c") ||
         starts_with_lang(value, "posix")) {
-        if (lang) *lang = LANG_EN;
+        if (lang) *lang = UI_LANG_EN;
         return true;
     }
 
     return false;
 }
 
-help_lang_t i18n_parse_lang(const char *value, help_lang_t fallback) {
-    help_lang_t lang;
-    if (i18n_try_parse_lang(value, &lang)) {
+ui_lang_t i18n_parse_ui_lang(const char *value, ui_lang_t fallback) {
+    ui_lang_t lang;
+    if (i18n_try_parse_ui_lang(value, &lang)) {
         return lang;
     }
     return fallback;
 }
 
-help_lang_t i18n_default_lang(void) {
+ui_lang_t i18n_default_ui_lang(void) {
     const char *explicit_lang = getenv("TNT_LANG");
     if (explicit_lang && explicit_lang[0] != '\0') {
-        return i18n_parse_lang(explicit_lang, LANG_EN);
+        return i18n_parse_ui_lang(explicit_lang, UI_LANG_EN);
     }
 
     const char *locale = getenv("LC_ALL");
@@ -78,15 +78,15 @@ help_lang_t i18n_default_lang(void) {
         locale = getenv("LANG");
     }
 
-    return i18n_parse_lang(locale, LANG_EN);
+    return i18n_parse_ui_lang(locale, UI_LANG_EN);
 }
 
-const char *i18n_lang_code(help_lang_t lang) {
-    return lang == LANG_ZH ? "zh" : "en";
+const char *i18n_ui_lang_code(ui_lang_t lang) {
+    return lang == UI_LANG_ZH ? "zh" : "en";
 }
 
-const char *i18n_text(help_lang_t lang, i18n_text_id_t id) {
-    if (lang == LANG_ZH) {
+const char *i18n_text(ui_lang_t lang, i18n_text_id_t id) {
+    if (lang == UI_LANG_ZH) {
         switch (id) {
             case I18N_USERNAME_PROMPT:
                 return "  请输入用户名 (留空 anonymous): ";
diff --git a/src/input.c b/src/input.c
index b9d2ca1..6172a5f 100644
--- a/src/input.c
+++ b/src/input.c
@@ -21,11 +21,11 @@
 #include <time.h>
 
 static int g_idle_timeout = DEFAULT_IDLE_TIMEOUT;
-static help_lang_t g_default_lang = LANG_EN;
+static ui_lang_t g_default_ui_lang = UI_LANG_EN;
 
 void input_init(void) {
     g_idle_timeout = env_int("TNT_IDLE_TIMEOUT", DEFAULT_IDLE_TIMEOUT, 0, 86400);
-    g_default_lang = i18n_default_lang();
+    g_default_ui_lang = i18n_default_ui_lang();
 }
 
 static int read_username(client_t *client) {
@@ -34,7 +34,7 @@ static int read_username(client_t *client) {
     char buf[4];
 
     tui_render_welcome(client);
-    client_printf(client, "%s", i18n_text(client->help_lang,
+    client_printf(client, "%s", i18n_text(client->ui_lang,
                                           I18N_USERNAME_PROMPT));
 
     while (1) {
@@ -117,7 +117,7 @@ static int read_username(client_t *client) {
 
         /* Validate username for security */
         if (!is_valid_username(client->username)) {
-            client_printf(client, "%s", i18n_text(client->help_lang,
+            client_printf(client, "%s", i18n_text(client->ui_lang,
                                                   I18N_INVALID_USERNAME));
             strcpy(client->username, "anonymous");
         } else {
@@ -258,11 +258,11 @@ static bool handle_key(client_t *client, unsigned char key, char *input) {
             client->show_help = false;
             tui_render_screen(client);
         } else if (key == 'e' || key == 'E') {
-            client->help_lang = LANG_EN;
+            client->ui_lang = UI_LANG_EN;
             client->help_scroll_pos = 0;
             tui_render_help(client);
         } else if (key == 'z' || key == 'Z') {
-            client->help_lang = LANG_ZH;
+            client->ui_lang = UI_LANG_ZH;
             client->help_scroll_pos = 0;
             tui_render_help(client);
         } else if (key == 'j') {
@@ -726,7 +726,7 @@ void input_run_session(client_t *client) {
     /* Terminal size already set from PTY request */
     client->mode = MODE_INSERT;
     client->follow_tail = true;
-    client->help_lang = g_default_lang;
+    client->ui_lang = g_default_ui_lang;
     client->connected = true;
     client->command_history_count = 0;
     client->command_history_pos = 0;
@@ -751,7 +751,7 @@ void input_run_session(client_t *client) {
 
     /* Add to room */
     if (room_add_client(g_room, client) < 0) {
-        client_printf(client, "%s", i18n_text(client->help_lang,
+        client_printf(client, "%s", i18n_text(client->ui_lang,
                                               I18N_ROOM_FULL));
         goto cleanup;
     }
@@ -765,7 +765,7 @@ void input_run_session(client_t *client) {
 
     /* Broadcast join message */
     message_t join_msg;
-    system_message_make_join(&join_msg, client->username, client->help_lang);
+    system_message_make_join(&join_msg, client->username, client->ui_lang);
     room_broadcast(g_room, &join_msg);
     message_save(&join_msg);
 
@@ -851,7 +851,7 @@ main_loop:
             if (g_idle_timeout > 0 && joined_room &&
                 time(NULL) - client->last_active >= g_idle_timeout) {
                 client_printf(client,
-                              i18n_text(client->help_lang,
+                              i18n_text(client->ui_lang,
                                         I18N_IDLE_TIMEOUT_FORMAT),
                               g_idle_timeout / 60);
                 break;
@@ -956,7 +956,7 @@ cleanup:
     if (joined_room) {
         message_t leave_msg;
         system_message_make_leave(&leave_msg, client->username,
-                                  client->help_lang);
+                                  client->ui_lang);
 
         client->connected = false;
         room_remove_client(g_room, client);
diff --git a/src/main.c b/src/main.c
index 983941b..947b1d2 100644
--- a/src/main.c
+++ b/src/main.c
@@ -20,7 +20,7 @@ static void signal_handler(int sig) {
 
 int main(int argc, char **argv) {
     int port = DEFAULT_PORT;
-    help_lang_t lang = i18n_default_lang();
+    ui_lang_t lang = i18n_default_ui_lang();
 
     /* Environment provides defaults; command-line flags override it. */
     const char *port_env = getenv("PORT");
diff --git a/src/manual.c b/src/manual.c
index 3d4ec43..da8a2d4 100644
--- a/src/manual.c
+++ b/src/manual.c
@@ -2,7 +2,7 @@
 #include "manual_text.h"
 
 void manual_append_interactive_panel(char *buffer, size_t buf_size,
-                                     size_t *pos, help_lang_t lang) {
+                                     size_t *pos, ui_lang_t lang) {
     if (!buffer || !pos) return;
 
     manual_text_append_interactive(buffer, buf_size, pos, lang);
diff --git a/src/manual_text.c b/src/manual_text.c
index d847fc0..b066e64 100644
--- a/src/manual_text.c
+++ b/src/manual_text.c
@@ -3,8 +3,8 @@
 #include "command_catalog.h"
 
 void manual_text_append_interactive(char *buffer, size_t buf_size,
-                                    size_t *pos, help_lang_t lang) {
-    if (lang == LANG_ZH) {
+                                    size_t *pos, ui_lang_t lang) {
+    if (lang == UI_LANG_ZH) {
         buffer_appendf(buffer, buf_size, pos,
                        "\033[1;36mTNT(1) 帮助\033[0m\n"
                        "\n"
diff --git a/src/system_message.c b/src/system_message.c
index 930b1d2..8ffd4d2 100644
--- a/src/system_message.c
+++ b/src/system_message.c
@@ -4,7 +4,7 @@
 #include <string.h>
 #include <time.h>
 
-static void system_message_init(message_t *msg, help_lang_t lang) {
+static void system_message_init(message_t *msg, ui_lang_t lang) {
     if (!msg) {
         return;
     }
@@ -16,7 +16,7 @@ static void system_message_init(message_t *msg, help_lang_t lang) {
 }
 
 void system_message_make_join(message_t *msg, const char *username,
-                              help_lang_t lang) {
+                              ui_lang_t lang) {
     system_message_init(msg, lang);
     if (!msg) {
         return;
@@ -28,7 +28,7 @@ void system_message_make_join(message_t *msg, const char *username,
 }
 
 void system_message_make_leave(message_t *msg, const char *username,
-                               help_lang_t lang) {
+                               ui_lang_t lang) {
     system_message_init(msg, lang);
     if (!msg) {
         return;
@@ -40,7 +40,7 @@ void system_message_make_leave(message_t *msg, const char *username,
 }
 
 void system_message_make_nick(message_t *msg, const char *old_name,
-                              const char *new_name, help_lang_t lang) {
+                              const char *new_name, ui_lang_t lang) {
     system_message_init(msg, lang);
     if (!msg) {
         return;
diff --git a/src/tui.c b/src/tui.c
index a59393a..1330ac7 100644
--- a/src/tui.c
+++ b/src/tui.c
@@ -154,8 +154,8 @@ void tui_render_welcome(client_t *client) {
 
     /* Lines, in display order.  Width is computed in display columns. */
     const char *line1 = "TNT · " TNT_VERSION;
-    const char *line2 = i18n_text(client->help_lang, I18N_WELCOME_SUBTITLE);
-    const char *line3 = i18n_text(client->help_lang, I18N_WELCOME_TAGLINE);
+    const char *line2 = i18n_text(client->ui_lang, I18N_WELCOME_SUBTITLE);
+    const char *line3 = i18n_text(client->ui_lang, I18N_WELCOME_TAGLINE);
 
     int inner_w = utf8_string_width(line1);
     int w2 = utf8_string_width(line2);
@@ -169,7 +169,7 @@ void tui_render_welcome(client_t *client) {
         char fallback_text[96];
         char fallback[128];
         snprintf(fallback_text, sizeof(fallback_text),
-                 i18n_text(client->help_lang, I18N_WELCOME_FALLBACK_FORMAT),
+                 i18n_text(client->ui_lang, I18N_WELCOME_FALLBACK_FORMAT),
                  TNT_VERSION);
         int n = snprintf(fallback, sizeof(fallback), ANSI_CLEAR ANSI_HOME "%s",
                          fallback_text);
@@ -355,7 +355,7 @@ void tui_render_screen(client_t *client) {
 
     char online_buf[32];
     snprintf(online_buf, sizeof(online_buf),
-             i18n_text(client->help_lang, I18N_TITLE_ONLINE_FORMAT),
+             i18n_text(client->ui_lang, I18N_TITLE_ONLINE_FORMAT),
              online);
     chips[chip_count].value = online_buf;
     chips[chip_count].value_color = "\033[37m";
@@ -373,9 +373,9 @@ void tui_render_screen(client_t *client) {
     chips[chip_count].value_color = mode_color;
     chip_count++;
 
-    const char *hint = i18n_text(client->help_lang, I18N_TITLE_HELP_HINT);
+    const char *hint = i18n_text(client->ui_lang, I18N_TITLE_HELP_HINT);
     int hint_width = utf8_string_width(hint);
-    const char *mute_label = i18n_text(client->help_lang, I18N_TITLE_MUTED);
+    const char *mute_label = i18n_text(client->ui_lang, I18N_TITLE_MUTED);
     int mute_width = client->mute_joins ? utf8_string_width(mute_label) + 2 : 0;
 
     /* Unread @-mentions chip — high-priority, gets a bright yellow star.
@@ -623,7 +623,7 @@ void tui_render_command_output(client_t *client) {
     buffer_appendf(buffer, sizeof(buffer), &pos, ANSI_CLEAR ANSI_HOME);
 
     /* Title */
-    const char *title = i18n_text(client->help_lang,
+    const char *title = i18n_text(client->ui_lang,
                                   I18N_COMMAND_OUTPUT_TITLE);
     char title_display[64];
     utf8_ansi_truncate(title, title_display, sizeof(title_display), rw);
@@ -676,7 +676,7 @@ void tui_render_command_output(client_t *client) {
     }
 
     buffer_appendf(buffer, sizeof(buffer), &pos,
-                   i18n_text(client->help_lang,
+                   i18n_text(client->ui_lang,
                              I18N_COMMAND_OUTPUT_STATUS_FORMAT),
                    start + 1, max_scroll + 1);
 
@@ -706,7 +706,7 @@ void tui_render_motd(client_t *client) {
     buffer_appendf(buffer, sizeof(buffer), &pos, ANSI_CLEAR ANSI_HOME);
 
     /* Top border with a localized title chip. */
-    const char *title = i18n_text(client->help_lang, I18N_MOTD_TITLE);
+    const char *title = i18n_text(client->ui_lang, I18N_MOTD_TITLE);
     int title_w = utf8_string_width(title);
     int top_dash_fill = rw - 2 - title_w - 1;  /* 2 corners, 1 leading ─ */
     if (top_dash_fill < 0) top_dash_fill = 0;
@@ -758,7 +758,7 @@ void tui_render_motd(client_t *client) {
     buffer_appendf(buffer, sizeof(buffer), &pos, "\r\n");
 
     /* Bottom border with a localized continue hint. */
-    const char *footer = i18n_text(client->help_lang,
+    const char *footer = i18n_text(client->ui_lang,
                                    I18N_MOTD_CONTINUE_HINT);
     int footer_w = utf8_string_width(footer);
     int bot_dash_fill = rw - 2 - footer_w - 1;
@@ -790,7 +790,7 @@ void tui_render_help(client_t *client) {
     buffer_appendf(buffer, sizeof(buffer), &pos, ANSI_CLEAR ANSI_HOME);
 
     /* Title */
-    const char *title = i18n_text(client->help_lang, I18N_HELP_TITLE);
+    const char *title = i18n_text(client->ui_lang, I18N_HELP_TITLE);
     int title_width = utf8_string_width(title);
     int padding = rw - title_width;
     if (padding < 0) padding = 0;
@@ -805,7 +805,7 @@ void tui_render_help(client_t *client) {
     size_t help_pos = 0;
     help_copy[0] = '\0';
     help_text_append_full(help_copy, sizeof(help_copy), &help_pos,
-                          client->help_lang);
+                          client->ui_lang);
 
     /* Split into lines and display with scrolling */
     char *lines[100];
@@ -836,7 +836,7 @@ void tui_render_help(client_t *client) {
 
     /* Status line */
     buffer_appendf(buffer, sizeof(buffer), &pos,
-                   i18n_text(client->help_lang, I18N_HELP_STATUS_FORMAT),
+                   i18n_text(client->ui_lang, I18N_HELP_STATUS_FORMAT),
                    start + 1, max_scroll + 1);
 
     client_send(client, buffer, pos);
diff --git a/src/tui_status.c b/src/tui_status.c
index 795b490..ed3b851 100644
--- a/src/tui_status.c
+++ b/src/tui_status.c
@@ -13,13 +13,13 @@ void tui_status_append(char *buffer, size_t buf_size, size_t *pos,
                            "\033[2;37m›\033[0m  "
                            "\033[2;37m%s\033[0m"
                            "\033[K",
-                           i18n_text(client->help_lang,
+                           i18n_text(client->ui_lang,
                                      I18N_INSERT_HINT_WIDE));
         } else if (client->width >= 36) {
             buffer_appendf(buffer, buf_size, pos,
                            "\033[2;37m›\033[0m  "
                            "\033[2;37m%s\033[0m\033[K",
-                           i18n_text(client->help_lang,
+                           i18n_text(client->ui_lang,
                                      I18N_INSERT_HINT_NARROW));
         } else {
             buffer_appendf(buffer, buf_size, pos, "\033[2;37m›\033[0m \033[K");
@@ -36,16 +36,16 @@ void tui_status_append(char *buffer, size_t buf_size, size_t *pos,
                            "  \033[2;37m%d-%d / %d\033[0m"
                            "   \033[33m▼ %d %s · %s\033[0m\033[K",
                            range_start, range_end, total, unseen,
-                           i18n_text(client->help_lang,
+                           i18n_text(client->ui_lang,
                                      I18N_NORMAL_NEW_MESSAGES),
-                           i18n_text(client->help_lang, I18N_NORMAL_LATEST));
+                           i18n_text(client->ui_lang, I18N_NORMAL_LATEST));
         } else {
             buffer_appendf(buffer, buf_size, pos,
                            "\033[7;33m NORMAL \033[0m"
                            "  \033[2;37m%d-%d / %d\033[0m"
                            "   \033[2;37m%s\033[0m\033[K",
                            range_start, range_end, total,
-                           i18n_text(client->help_lang, I18N_NORMAL_LATEST));
+                           i18n_text(client->ui_lang, I18N_NORMAL_LATEST));
         }
     } else if (client->mode == MODE_COMMAND) {
         buffer_appendf(buffer, buf_size, pos,
diff --git a/tests/unit/test_cli_text.c b/tests/unit/test_cli_text.c
index e650c00..ff972a6 100644
--- a/tests/unit/test_cli_text.c
+++ b/tests/unit/test_cli_text.c
@@ -19,31 +19,31 @@ TEST(help_matches_language) {
     char output[2048] = {0};
     size_t pos = 0;
 
-    cli_text_append_help(output, sizeof(output), &pos, "tnt", LANG_EN);
+    cli_text_append_help(output, sizeof(output), &pos, "tnt", UI_LANG_EN);
     assert(strstr(output, "anonymous SSH chat server") != NULL);
     assert(strstr(output, "Usage: tnt [options]") != NULL);
     assert(strstr(output, "TNT_LANG") != NULL);
 
     memset(output, 0, sizeof(output));
     pos = 0;
-    cli_text_append_help(output, sizeof(output), &pos, "tnt", LANG_ZH);
+    cli_text_append_help(output, sizeof(output), &pos, "tnt", UI_LANG_ZH);
     assert(strstr(output, "匿名 SSH 聊天服务器") != NULL);
     assert(strstr(output, "用法: tnt [选项]") != NULL);
     assert(strstr(output, "TNT_LANG") != NULL);
 }
 
 TEST(error_formats_match_language) {
-    assert(strcmp(cli_text_invalid_port_format(LANG_EN),
+    assert(strcmp(cli_text_invalid_port_format(UI_LANG_EN),
                   "Invalid port: %s\n") == 0);
-    assert(strcmp(cli_text_invalid_port_format(LANG_ZH),
+    assert(strcmp(cli_text_invalid_port_format(UI_LANG_ZH),
                   "端口无效: %s\n") == 0);
-    assert(strcmp(cli_text_unknown_option_format(LANG_EN),
+    assert(strcmp(cli_text_unknown_option_format(UI_LANG_EN),
                   "Unknown option: %s\n") == 0);
-    assert(strcmp(cli_text_unknown_option_format(LANG_ZH),
+    assert(strcmp(cli_text_unknown_option_format(UI_LANG_ZH),
                   "未知选项: %s\n") == 0);
-    assert(strcmp(cli_text_short_usage_format(LANG_EN),
+    assert(strcmp(cli_text_short_usage_format(UI_LANG_EN),
                   "Usage: %s [-p PORT] [-d DIR] [-h]\n") == 0);
-    assert(strcmp(cli_text_short_usage_format(LANG_ZH),
+    assert(strcmp(cli_text_short_usage_format(UI_LANG_ZH),
                   "用法: %s [-p PORT] [-d DIR] [-h]\n") == 0);
 }
 
diff --git a/tests/unit/test_command_catalog.c b/tests/unit/test_command_catalog.c
index 222889d..db05378 100644
--- a/tests/unit/test_command_catalog.c
+++ b/tests/unit/test_command_catalog.c
@@ -61,8 +61,8 @@ TEST(generates_localized_help_sections) {
     size_t en_pos = 0;
     size_t zh_pos = 0;
 
-    command_catalog_append_full(en, sizeof(en), &en_pos, LANG_EN);
-    command_catalog_append_full(zh, sizeof(zh), &zh_pos, LANG_ZH);
+    command_catalog_append_full(en, sizeof(en), &en_pos, UI_LANG_EN);
+    command_catalog_append_full(zh, sizeof(zh), &zh_pos, UI_LANG_ZH);
 
     assert(strstr(en, ":users, :list, :who") != NULL);
     assert(strstr(en, "Show online users") != NULL);
diff --git a/tests/unit/test_help_text.c b/tests/unit/test_help_text.c
index 4db040a..af51f6e 100644
--- a/tests/unit/test_help_text.c
+++ b/tests/unit/test_help_text.c
@@ -21,8 +21,8 @@ TEST(full_help_matches_language) {
     size_t en_pos = 0;
     size_t zh_pos = 0;
 
-    help_text_append_full(en, sizeof(en), &en_pos, LANG_EN);
-    help_text_append_full(zh, sizeof(zh), &zh_pos, LANG_ZH);
+    help_text_append_full(en, sizeof(en), &en_pos, UI_LANG_EN);
+    help_text_append_full(zh, sizeof(zh), &zh_pos, UI_LANG_ZH);
 
     assert(strstr(en, "TNT KEY REFERENCE") != NULL);
     assert(strstr(en, "AVAILABLE COMMANDS") != NULL);
diff --git a/tests/unit/test_i18n.c b/tests/unit/test_i18n.c
index f53ab15..8b11f1e 100644
--- a/tests/unit/test_i18n.c
+++ b/tests/unit/test_i18n.c
@@ -17,133 +17,133 @@
 static int tests_passed = 0;
 
 TEST(parse_explicit_languages) {
-    help_lang_t lang;
+    ui_lang_t lang;
 
-    assert(i18n_parse_lang("zh", LANG_EN) == LANG_ZH);
-    assert(i18n_parse_lang("zh_CN.UTF-8", LANG_EN) == LANG_ZH);
-    assert(i18n_parse_lang("en", LANG_ZH) == LANG_EN);
-    assert(i18n_parse_lang("en_US.UTF-8", LANG_ZH) == LANG_EN);
-    assert(i18n_parse_lang("C", LANG_ZH) == LANG_EN);
-    assert(i18n_parse_lang("POSIX", LANG_ZH) == LANG_EN);
+    assert(i18n_parse_ui_lang("zh", UI_LANG_EN) == UI_LANG_ZH);
+    assert(i18n_parse_ui_lang("zh_CN.UTF-8", UI_LANG_EN) == UI_LANG_ZH);
+    assert(i18n_parse_ui_lang("en", UI_LANG_ZH) == UI_LANG_EN);
+    assert(i18n_parse_ui_lang("en_US.UTF-8", UI_LANG_ZH) == UI_LANG_EN);
+    assert(i18n_parse_ui_lang("C", UI_LANG_ZH) == UI_LANG_EN);
+    assert(i18n_parse_ui_lang("POSIX", UI_LANG_ZH) == UI_LANG_EN);
 
-    assert(i18n_try_parse_lang("zh", &lang) == true);
-    assert(lang == LANG_ZH);
-    assert(i18n_try_parse_lang("en", &lang) == true);
-    assert(lang == LANG_EN);
-    assert(i18n_try_parse_lang("cn", &lang) == false);
-    assert(i18n_try_parse_lang("english", &lang) == false);
-    assert(i18n_try_parse_lang("chinese", &lang) == false);
-    assert(i18n_try_parse_lang("中文", &lang) == false);
-    assert(i18n_try_parse_lang("英文", &lang) == false);
-    assert(i18n_try_parse_lang("fr", &lang) == false);
+    assert(i18n_try_parse_ui_lang("zh", &lang) == true);
+    assert(lang == UI_LANG_ZH);
+    assert(i18n_try_parse_ui_lang("en", &lang) == true);
+    assert(lang == UI_LANG_EN);
+    assert(i18n_try_parse_ui_lang("cn", &lang) == false);
+    assert(i18n_try_parse_ui_lang("english", &lang) == false);
+    assert(i18n_try_parse_ui_lang("chinese", &lang) == false);
+    assert(i18n_try_parse_ui_lang("中文", &lang) == false);
+    assert(i18n_try_parse_ui_lang("英文", &lang) == false);
+    assert(i18n_try_parse_ui_lang("fr", &lang) == false);
 }
 
 TEST(parse_unknown_uses_fallback) {
-    assert(i18n_parse_lang(NULL, LANG_ZH) == LANG_ZH);
-    assert(i18n_parse_lang("", LANG_EN) == LANG_EN);
-    assert(i18n_parse_lang("fr_FR.UTF-8", LANG_ZH) == LANG_ZH);
+    assert(i18n_parse_ui_lang(NULL, UI_LANG_ZH) == UI_LANG_ZH);
+    assert(i18n_parse_ui_lang("", UI_LANG_EN) == UI_LANG_EN);
+    assert(i18n_parse_ui_lang("fr_FR.UTF-8", UI_LANG_ZH) == UI_LANG_ZH);
 }
 
 TEST(parse_ignores_surrounding_whitespace) {
-    help_lang_t lang;
+    ui_lang_t lang;
 
-    assert(i18n_try_parse_lang("  zh  ", &lang) == true);
-    assert(lang == LANG_ZH);
-    assert(i18n_parse_lang("\ten_US.UTF-8\n", LANG_ZH) == LANG_EN);
-    assert(i18n_try_parse_lang(" english ", &lang) == false);
-    assert(i18n_try_parse_lang("zh CN", &lang) == false);
+    assert(i18n_try_parse_ui_lang("  zh  ", &lang) == true);
+    assert(lang == UI_LANG_ZH);
+    assert(i18n_parse_ui_lang("\ten_US.UTF-8\n", UI_LANG_ZH) == UI_LANG_EN);
+    assert(i18n_try_parse_ui_lang(" english ", &lang) == false);
+    assert(i18n_try_parse_ui_lang("zh CN", &lang) == false);
 
     setenv("TNT_LANG", " zh ", 1);
     setenv("LC_ALL", "en_US.UTF-8", 1);
-    assert(i18n_default_lang() == LANG_ZH);
+    assert(i18n_default_ui_lang() == UI_LANG_ZH);
 }
 
 TEST(default_prefers_tnt_lang) {
     setenv("TNT_LANG", "zh_CN.UTF-8", 1);
     setenv("LC_ALL", "en_US.UTF-8", 1);
-    assert(i18n_default_lang() == LANG_ZH);
+    assert(i18n_default_ui_lang() == UI_LANG_ZH);
 
     setenv("TNT_LANG", "en", 1);
     setenv("LC_ALL", "zh_CN.UTF-8", 1);
-    assert(i18n_default_lang() == LANG_EN);
+    assert(i18n_default_ui_lang() == UI_LANG_EN);
 }
 
 TEST(default_uses_locale_when_no_tnt_lang) {
     unsetenv("TNT_LANG");
     setenv("LC_ALL", "zh_CN.UTF-8", 1);
-    assert(i18n_default_lang() == LANG_ZH);
+    assert(i18n_default_ui_lang() == UI_LANG_ZH);
 
     setenv("LC_ALL", "C", 1);
-    assert(i18n_default_lang() == LANG_EN);
+    assert(i18n_default_ui_lang() == UI_LANG_EN);
 }
 
 TEST(text_lookup_matches_language) {
-    assert(strstr(i18n_text(LANG_EN, I18N_USERNAME_PROMPT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_USERNAME_PROMPT),
                   "display name") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_USERNAME_PROMPT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_USERNAME_PROMPT),
                   "用户名") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_WELCOME_SUBTITLE),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_WELCOME_SUBTITLE),
                   "anonymous chat") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_WELCOME_SUBTITLE),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_WELCOME_SUBTITLE),
                   "匿名聊天室") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_HELP_STATUS_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_HELP_STATUS_FORMAT),
                   "KEY REFERENCE") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_HELP_STATUS_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_HELP_STATUS_FORMAT),
                   "按键参考") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_COMMAND_OUTPUT_TITLE),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_COMMAND_OUTPUT_TITLE),
                   "COMMAND") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_COMMAND_OUTPUT_TITLE),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_COMMAND_OUTPUT_TITLE),
                   "命令输出") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_COMMAND_OUTPUT_STATUS_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_COMMAND_OUTPUT_STATUS_FORMAT),
                   "q:close") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_COMMAND_OUTPUT_STATUS_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_COMMAND_OUTPUT_STATUS_FORMAT),
                   "q:关闭") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_MOTD_CONTINUE_HINT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_MOTD_CONTINUE_HINT),
                   "Press any key") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_MOTD_CONTINUE_HINT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_MOTD_CONTINUE_HINT),
                   "按任意键") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_TITLE_ONLINE_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_TITLE_ONLINE_FORMAT),
                   "online") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_TITLE_ONLINE_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_TITLE_ONLINE_FORMAT),
                   "在线") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_IDLE_TIMEOUT_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_IDLE_TIMEOUT_FORMAT),
                   "idle timeout") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_IDLE_TIMEOUT_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_IDLE_TIMEOUT_FORMAT),
                   "空闲超时") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_MSG_USAGE),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_MSG_USAGE),
                   "msg <username>") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_MSG_USAGE),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_MSG_USAGE),
                   "用户名") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_SEARCH_HEADER_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_SEARCH_HEADER_FORMAT),
                   "Search") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_SEARCH_HEADER_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_SEARCH_HEADER_FORMAT),
                   "搜索") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_LANG_CURRENT_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_LANG_CURRENT_FORMAT),
                   "lang <en|zh>") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_LANG_CURRENT_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_LANG_CURRENT_FORMAT),
                   "lang <en|zh>") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_UNKNOWN_COMMAND_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_UNKNOWN_COMMAND_FORMAT),
                   "Unknown command") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_UNKNOWN_COMMAND_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_UNKNOWN_COMMAND_FORMAT),
                   "未知命令") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_EXEC_HELP),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_EXEC_HELP),
                   "TNT exec interface") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_EXEC_HELP),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_EXEC_HELP),
                   "support") == NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_EXEC_HELP),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_EXEC_HELP),
                   "TNT exec 接口") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_EXEC_HELP),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_EXEC_HELP),
                   "support") == NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_EXEC_POST_EMPTY),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_EXEC_POST_EMPTY),
                   "message cannot be empty") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_EXEC_POST_EMPTY),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_EXEC_POST_EMPTY),
                   "消息不能为空") != NULL);
-    assert(strstr(i18n_text(LANG_EN, I18N_EXEC_UNKNOWN_COMMAND_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_EN, I18N_EXEC_UNKNOWN_COMMAND_FORMAT),
                   "Unknown command") != NULL);
-    assert(strstr(i18n_text(LANG_ZH, I18N_EXEC_UNKNOWN_COMMAND_FORMAT),
+    assert(strstr(i18n_text(UI_LANG_ZH, I18N_EXEC_UNKNOWN_COMMAND_FORMAT),
                   "未知命令") != NULL);
-    assert(strcmp(i18n_lang_code(LANG_EN), "en") == 0);
-    assert(strcmp(i18n_lang_code(LANG_ZH), "zh") == 0);
+    assert(strcmp(i18n_ui_lang_code(UI_LANG_EN), "en") == 0);
+    assert(strcmp(i18n_ui_lang_code(UI_LANG_ZH), "zh") == 0);
 }
 
 int main(void) {
diff --git a/tests/unit/test_manual_text.c b/tests/unit/test_manual_text.c
index 870dde6..4405b77 100644
--- a/tests/unit/test_manual_text.c
+++ b/tests/unit/test_manual_text.c
@@ -34,8 +34,8 @@ TEST(interactive_manual_matches_language) {
     size_t en_pos = 0;
     size_t zh_pos = 0;
 
-    manual_text_append_interactive(en, sizeof(en), &en_pos, LANG_EN);
-    manual_text_append_interactive(zh, sizeof(zh), &zh_pos, LANG_ZH);
+    manual_text_append_interactive(en, sizeof(en), &en_pos, UI_LANG_EN);
+    manual_text_append_interactive(zh, sizeof(zh), &zh_pos, UI_LANG_ZH);
 
     assert(strstr(en, "TNT(1) help") != NULL);
     assert(strstr(en, "Use") != NULL);
diff --git a/tests/unit/test_system_message.c b/tests/unit/test_system_message.c
index 1319170..9ee6050 100644
--- a/tests/unit/test_system_message.c
+++ b/tests/unit/test_system_message.c
@@ -18,14 +18,14 @@ static int tests_passed = 0;
 TEST(join_leave_follow_language) {
     message_t msg;
 
-    system_message_make_join(&msg, "alice", LANG_ZH);
+    system_message_make_join(&msg, "alice", UI_LANG_ZH);
     assert(strcmp(msg.username, "系统") == 0);
     assert(strstr(msg.content, "alice") != NULL);
     assert(strstr(msg.content, "加入了聊天室") != NULL);
     assert(system_message_is_system(&msg));
     assert(system_message_is_join_leave(&msg));
 
-    system_message_make_leave(&msg, "bob", LANG_EN);
+    system_message_make_leave(&msg, "bob", UI_LANG_EN);
     assert(strcmp(msg.username, "system") == 0);
     assert(strstr(msg.content, "bob") != NULL);
     assert(strstr(msg.content, "left the room") != NULL);
@@ -36,7 +36,7 @@ TEST(join_leave_follow_language) {
 TEST(nick_messages_are_system_events_not_join_leave) {
     message_t msg;
 
-    system_message_make_nick(&msg, "old", "new", LANG_EN);
+    system_message_make_nick(&msg, "old", "new", UI_LANG_EN);
     assert(strcmp(msg.username, "system") == 0);
     assert(strstr(msg.content, "old") != NULL);
     assert(strstr(msg.content, "new") != NULL);
@@ -44,7 +44,7 @@ TEST(nick_messages_are_system_events_not_join_leave) {
     assert(system_message_is_system(&msg));
     assert(!system_message_is_join_leave(&msg));
 
-    system_message_make_nick(&msg, "旧", "新", LANG_ZH);
+    system_message_make_nick(&msg, "旧", "新", UI_LANG_ZH);
     assert(strcmp(msg.username, "系统") == 0);
     assert(strstr(msg.content, "更名为") != NULL);
     assert(system_message_is_system(&msg));