diff --git a/.gitignore b/.gitignore
index ce4bc6c..f5f5f5f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,3 +12,4 @@ tests/unit/test_utf8
 tests/unit/test_message
 tests/unit/test_chat_room
 tests/unit/test_history_view
+tests/unit/test_i18n
diff --git a/README.md b/README.md
index 864320b..aaa158c 100644
--- a/README.md
+++ b/README.md
@@ -127,6 +127,9 @@ TNT_STATE_DIR=/var/lib/tnt tnt
 
 # Show the public SSH endpoint in startup logs
 TNT_PUBLIC_HOST=chat.m1ng.space tnt
+
+# Choose interactive UI language (en or zh; defaults from locale)
+TNT_LANG=zh tnt
 ```
 
 **Rate limiting:**
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 6718561..5edd4a9 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 ## 2026-05-21 - Message browsing polish
 
+### 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`.
+
 ### Changed
 - NORMAL mode now opens at the latest visible messages instead of the oldest
   in-memory message. Use `k`/PageUp to browse older history and `G`/End to
diff --git a/include/i18n.h b/include/i18n.h
new file mode 100644
index 0000000..f1d20db
--- /dev/null
+++ b/include/i18n.h
@@ -0,0 +1,21 @@
+#ifndef I18N_H
+#define I18N_H
+
+#include "common.h"
+
+typedef enum {
+    I18N_USERNAME_PROMPT,
+    I18N_INVALID_USERNAME,
+    I18N_ROOM_FULL,
+    I18N_INSERT_HINT_WIDE,
+    I18N_INSERT_HINT_NARROW,
+    I18N_NORMAL_LATEST,
+    I18N_NORMAL_NEW_MESSAGES
+} i18n_text_id_t;
+
+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);
+
+#endif /* I18N_H */
diff --git a/include/support.h b/include/support.h
index 4768aa4..cabc9b4 100644
--- a/include/support.h
+++ b/include/support.h
@@ -4,7 +4,8 @@
 #include "common.h"
 
 void support_append_interactive_panel(char *buffer, size_t buf_size,
-                                      size_t *pos);
-void support_append_exec_panel(char *buffer, size_t buf_size, size_t *pos);
+                                      size_t *pos, help_lang_t lang);
+void support_append_exec_panel(char *buffer, size_t buf_size, size_t *pos,
+                               help_lang_t lang);
 
 #endif /* SUPPORT_H */
diff --git a/src/commands.c b/src/commands.c
index dffd56f..b20f79a 100644
--- a/src/commands.c
+++ b/src/commands.c
@@ -188,7 +188,8 @@ void commands_dispatch(client_t *client) {
                        "========================================\n");
 
     } else if (strcmp(cmd, "support") == 0 || strcmp(cmd, "guide") == 0) {
-        support_append_interactive_panel(output, sizeof(output), &pos);
+        support_append_interactive_panel(output, sizeof(output), &pos,
+                                         client->help_lang);
 
     } else if (strncmp(cmd, "msg ", 4) == 0 || strncmp(cmd, "w ", 2) == 0) {
         char *rest = (cmd[0] == 'w') ? cmd + 2 : cmd + 4;
diff --git a/src/exec.c b/src/exec.c
index 3df1d3d..37dfd37 100644
--- a/src/exec.c
+++ b/src/exec.c
@@ -137,7 +137,7 @@ static int exec_command_support(client_t *client) {
     char output[2048] = {0};
     size_t pos = 0;
 
-    support_append_exec_panel(output, sizeof(output), &pos);
+    support_append_exec_panel(output, sizeof(output), &pos, client->help_lang);
     return client_send(client, output, pos) == 0 ? 0 : 1;
 }
 
diff --git a/src/i18n.c b/src/i18n.c
new file mode 100644
index 0000000..4157050
--- /dev/null
+++ b/src/i18n.c
@@ -0,0 +1,99 @@
+#include "i18n.h"
+
+#include <ctype.h>
+
+static bool starts_with_lang(const char *value, const char *prefix) {
+    if (!value || !prefix) return false;
+
+    while (*prefix) {
+        if (tolower((unsigned char)*value) !=
+            tolower((unsigned char)*prefix)) {
+            return false;
+        }
+        value++;
+        prefix++;
+    }
+
+    return *value == '\0' || *value == '_' || *value == '-' || *value == '.';
+}
+
+help_lang_t i18n_parse_lang(const char *value, help_lang_t fallback) {
+    if (!value || value[0] == '\0') {
+        return fallback;
+    }
+
+    if (starts_with_lang(value, "zh") ||
+        starts_with_lang(value, "cn") ||
+        starts_with_lang(value, "chinese")) {
+        return LANG_ZH;
+    }
+
+    if (starts_with_lang(value, "en") ||
+        starts_with_lang(value, "c") ||
+        starts_with_lang(value, "posix")) {
+        return LANG_EN;
+    }
+
+    return fallback;
+}
+
+help_lang_t i18n_default_lang(void) {
+    const char *explicit_lang = getenv("TNT_LANG");
+    if (explicit_lang && explicit_lang[0] != '\0') {
+        return i18n_parse_lang(explicit_lang, LANG_EN);
+    }
+
+    const char *locale = getenv("LC_ALL");
+    if (!locale || locale[0] == '\0') {
+        locale = getenv("LC_MESSAGES");
+    }
+    if (!locale || locale[0] == '\0') {
+        locale = getenv("LANG");
+    }
+
+    return i18n_parse_lang(locale, LANG_EN);
+}
+
+const char *i18n_lang_code(help_lang_t lang) {
+    return lang == LANG_ZH ? "zh" : "en";
+}
+
+const char *i18n_text(help_lang_t lang, i18n_text_id_t id) {
+    if (lang == LANG_ZH) {
+        switch (id) {
+            case I18N_USERNAME_PROMPT:
+                return "  请输入用户名 (留空 anonymous): ";
+            case I18N_INVALID_USERNAME:
+                return "用户名无效，已改用 anonymous。\r\n";
+            case I18N_ROOM_FULL:
+                return "房间已满\r\n";
+            case I18N_INSERT_HINT_WIDE:
+                return "Enter 发送 · Esc 浏览 · :support";
+            case I18N_INSERT_HINT_NARROW:
+                return "Enter · Esc · :support";
+            case I18N_NORMAL_LATEST:
+                return "G 最新";
+            case I18N_NORMAL_NEW_MESSAGES:
+                return "新消息";
+        }
+    }
+
+    switch (id) {
+        case I18N_USERNAME_PROMPT:
+            return "  Enter display name (blank for anonymous): ";
+        case I18N_INVALID_USERNAME:
+            return "Invalid username. Using 'anonymous' instead.\r\n";
+        case I18N_ROOM_FULL:
+            return "Room is full\r\n";
+        case I18N_INSERT_HINT_WIDE:
+            return "Enter send · Esc browse · :support";
+        case I18N_INSERT_HINT_NARROW:
+            return "Enter · Esc · :support";
+        case I18N_NORMAL_LATEST:
+            return "G latest";
+        case I18N_NORMAL_NEW_MESSAGES:
+            return "new";
+    }
+
+    return "";
+}
diff --git a/src/input.c b/src/input.c
index bec45a1..707faea 100644
--- a/src/input.c
+++ b/src/input.c
@@ -5,6 +5,7 @@
 #include "common.h"
 #include "exec.h"
 #include "history_view.h"
+#include "i18n.h"
 #include "message.h"
 #include "ratelimit.h"
 #include "tui.h"
@@ -19,9 +20,11 @@
 #include <time.h>
 
 static int g_idle_timeout = DEFAULT_IDLE_TIMEOUT;
+static help_lang_t g_default_lang = 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();
 }
 
 static int read_username(client_t *client) {
@@ -30,7 +33,8 @@ static int read_username(client_t *client) {
     char buf[4];
 
     tui_render_welcome(client);
-    client_printf(client, "  请输入用户名 (留空 anonymous): ");
+    client_printf(client, "%s", i18n_text(client->help_lang,
+                                          I18N_USERNAME_PROMPT));
 
     while (1) {
         int n = ssh_channel_read_timeout(client->channel, buf, 1, 0, 60000); /* 60 sec timeout */
@@ -112,7 +116,8 @@ static int read_username(client_t *client) {
 
         /* Validate username for security */
         if (!is_valid_username(client->username)) {
-            client_printf(client, "Invalid username. Using 'anonymous' instead.\r\n");
+            client_printf(client, "%s", i18n_text(client->help_lang,
+                                                  I18N_INVALID_USERNAME));
             strcpy(client->username, "anonymous");
         } else {
             /* Truncate to 20 characters */
@@ -659,7 +664,7 @@ 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 = LANG_ZH;
+    client->help_lang = g_default_lang;
     client->connected = true;
     client->command_history_count = 0;
     client->command_history_pos = 0;
@@ -683,7 +688,8 @@ void input_run_session(client_t *client) {
 
     /* Add to room */
     if (room_add_client(g_room, client) < 0) {
-        client_printf(client, "Room is full\n");
+        client_printf(client, "%s", i18n_text(client->help_lang,
+                                              I18N_ROOM_FULL));
         goto cleanup;
     }
     joined_room = true;
diff --git a/src/main.c b/src/main.c
index 175ee42..16be8b8 100644
--- a/src/main.c
+++ b/src/main.c
@@ -63,6 +63,7 @@ int main(int argc, char **argv) {
             printf("  PORT                  Default listening port\n");
             printf("  TNT_STATE_DIR         State directory\n");
             printf("  TNT_ACCESS_TOKEN      Require this password for SSH auth\n");
+            printf("  TNT_LANG              UI language: en or zh (default: locale)\n");
             printf("  TNT_MAX_CONNECTIONS   Global connection limit (default: 64)\n");
             printf("  TNT_RATE_LIMIT        Set to 0 to disable rate limiting\n");
             printf("  TNT_IDLE_TIMEOUT      Idle disconnect timeout in seconds (default: 1800)\n");
diff --git a/src/support.c b/src/support.c
index 3bc55bb..a85b842 100644
--- a/src/support.c
+++ b/src/support.c
@@ -1,39 +1,92 @@
 #include "support.h"
 
 void support_append_interactive_panel(char *buffer, size_t buf_size,
-                                      size_t *pos) {
+                                      size_t *pos, help_lang_t lang) {
     if (!buffer || !pos) return;
 
+    if (lang == LANG_ZH) {
+        buffer_appendf(buffer, buf_size, pos,
+                       "\033[1;36m支持 · support\033[0m\n"
+                       "\n"
+                       "\033[1;37m第一次进来\033[0m\n"
+                       "  INSERT  输入消息，Enter 发送，ESC 进入 NORMAL\n"
+                       "  NORMAL  浏览消息，G 回到最新，i 继续输入\n"
+                       "  COMMAND 按 : 输入命令，q/ESC 关闭当前面板\n"
+                       "\n"
+                       "\033[1;37m我想...\033[0m\n"
+                       "  看谁在线        :users\n"
+                       "  看最近历史      :last 20\n"
+                       "  搜索聊天记录    :search <keyword>\n"
+                       "  回到最新消息    G 或 End\n"
+                       "  私聊某个人      :msg <user> <text>\n"
+                       "  查看私聊收件箱  :inbox\n"
+                       "  静音进出提示    :mute-joins\n"
+                       "\n"
+                       "\033[1;37m遇到问题\033[0m\n"
+                       "  看不到新消息: 在 NORMAL 按 G 或 End 回到最新\n"
+                       "  粘贴多行文本: 直接粘贴，TNT 会等 Enter 后一次发送\n"
+                       "  输入太长: 状态行接近限制时会提示，超出会响铃\n"
+                       "  命令不记得: 输入 :help 看列表，输入 :support 回到这里\n"
+                       "  连接断开: 可能是空闲超时、连接数限制或网络重连\n"
+                       "\n"
+                       "\033[2;37m更多: ? 打开完整按键帮助，:help 查看命令列表\033[0m\n");
+        return;
+    }
+
     buffer_appendf(buffer, buf_size, pos,
-                   "\033[1;36m支持 · support\033[0m\n"
+                   "\033[1;36mSupport\033[0m\n"
                    "\n"
-                   "\033[1;37m第一次进来\033[0m\n"
-                   "  INSERT  输入消息，Enter 发送，ESC 进入 NORMAL\n"
-                   "  NORMAL  浏览消息，G 回到最新，i 继续输入\n"
-                   "  COMMAND 按 : 输入命令，q/ESC 关闭当前面板\n"
+                   "\033[1;37mFirst minute\033[0m\n"
+                   "  INSERT  Type messages, Enter sends, ESC enters NORMAL\n"
+                   "  NORMAL  Browse history, G jumps latest, i continues typing\n"
+                   "  COMMAND Press : for commands, q/ESC closes this panel\n"
                    "\n"
-                   "\033[1;37m我想...\033[0m\n"
-                   "  看谁在线        :users\n"
-                   "  看最近历史      :last 20\n"
-                   "  搜索聊天记录    :search <keyword>\n"
-                   "  回到最新消息    G 或 End\n"
-                   "  私聊某个人      :msg <user> <text>\n"
-                   "  查看私聊收件箱  :inbox\n"
-                   "  静音进出提示    :mute-joins\n"
+                   "\033[1;37mI want to...\033[0m\n"
+                   "  See who is online   :users\n"
+                   "  See recent history  :last 20\n"
+                   "  Search history      :search <keyword>\n"
+                   "  Return to latest    G or End\n"
+                   "  Whisper someone     :msg <user> <text>\n"
+                   "  Read whispers       :inbox\n"
+                   "  Mute join notices   :mute-joins\n"
                    "\n"
-                   "\033[1;37m遇到问题\033[0m\n"
-                   "  看不到新消息: 在 NORMAL 按 G 或 End 回到最新\n"
-                   "  粘贴多行文本: 直接粘贴，TNT 会等 Enter 后一次发送\n"
-                   "  输入太长: 状态行接近限制时会提示，超出会响铃\n"
-                   "  命令不记得: 输入 :help 看列表，输入 :support 回到这里\n"
-                   "  连接断开: 可能是空闲超时、连接数限制或网络重连\n"
+                   "\033[1;37mTroubleshooting\033[0m\n"
+                   "  Missing new messages: press G or End in NORMAL\n"
+                   "  Pasting many lines: paste normally, then Enter sends once\n"
+                   "  Message too long: the status line warns near the limit\n"
+                   "  Forgot a command: type :help or return here with :support\n"
+                   "  Disconnected: check idle timeout, limits, or reconnect\n"
                    "\n"
-                   "\033[2;37m更多: ? 打开完整按键帮助，:help 查看命令列表\033[0m\n");
+                   "\033[2;37mMore: ? opens full key help, :help lists commands\033[0m\n");
 }
 
-void support_append_exec_panel(char *buffer, size_t buf_size, size_t *pos) {
+void support_append_exec_panel(char *buffer, size_t buf_size, size_t *pos,
+                               help_lang_t lang) {
     if (!buffer || !pos) return;
 
+    if (lang == LANG_ZH) {
+        buffer_appendf(buffer, buf_size, pos,
+                       "TNT 支持\n"
+                       "\n"
+                       "交互使用:\n"
+                       "  ssh -p 2222 HOST\n"
+                       "  INSERT: 输入消息并按 Enter 发送\n"
+                       "  NORMAL: G 回到最新，k/PageUp 查看更早消息\n"
+                       "  COMMAND: 按 : 后可运行 users, last, search, msg, inbox\n"
+                       "\n"
+                       "非交互检查:\n"
+                       "  ssh -p 2222 HOST health\n"
+                       "  ssh -p 2222 HOST stats --json\n"
+                       "  ssh -p 2222 HOST users --json\n"
+                       "  ssh -p 2222 HOST 'tail -n 20'\n"
+                       "  ssh -p 2222 USER@HOST post 'message'\n"
+                       "\n"
+                       "排查:\n"
+                       "  连接过早关闭: 检查限流、空闲超时、连接容量、\n"
+                       "  单 IP 限制和防火墙规则。\n");
+        return;
+    }
+
     buffer_appendf(buffer, buf_size, pos,
                    "TNT support\n"
                    "\n"
diff --git a/src/tui_status.c b/src/tui_status.c
index 24e6d8b..795b490 100644
--- a/src/tui_status.c
+++ b/src/tui_status.c
@@ -1,4 +1,5 @@
 #include "tui_status.h"
+#include "i18n.h"
 #include "ssh_server.h"
 
 void tui_status_append(char *buffer, size_t buf_size, size_t *pos,
@@ -10,12 +11,16 @@ void tui_status_append(char *buffer, size_t buf_size, size_t *pos,
         if (client->width >= 58) {
             buffer_appendf(buffer, buf_size, pos,
                            "\033[2;37m›\033[0m  "
-                           "\033[2;37mEnter send · Esc browse · :support\033[0m"
-                           "\033[K");
+                           "\033[2;37m%s\033[0m"
+                           "\033[K",
+                           i18n_text(client->help_lang,
+                                     I18N_INSERT_HINT_WIDE));
         } else if (client->width >= 36) {
             buffer_appendf(buffer, buf_size, pos,
                            "\033[2;37m›\033[0m  "
-                           "\033[2;37mEnter · Esc · :support\033[0m\033[K");
+                           "\033[2;37m%s\033[0m\033[K",
+                           i18n_text(client->help_lang,
+                                     I18N_INSERT_HINT_NARROW));
         } else {
             buffer_appendf(buffer, buf_size, pos, "\033[2;37m›\033[0m \033[K");
         }
@@ -29,14 +34,18 @@ void tui_status_append(char *buffer, size_t buf_size, size_t *pos,
             buffer_appendf(buffer, buf_size, pos,
                            "\033[7;33m NORMAL \033[0m"
                            "  \033[2;37m%d-%d / %d\033[0m"
-                           "   \033[33m▼ %d new · G latest\033[0m\033[K",
-                           range_start, range_end, total, unseen);
+                           "   \033[33m▼ %d %s · %s\033[0m\033[K",
+                           range_start, range_end, total, unseen,
+                           i18n_text(client->help_lang,
+                                     I18N_NORMAL_NEW_MESSAGES),
+                           i18n_text(client->help_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;37mG latest\033[0m\033[K",
-                           range_start, range_end, total);
+                           "   \033[2;37m%s\033[0m\033[K",
+                           range_start, range_end, total,
+                           i18n_text(client->help_lang, I18N_NORMAL_LATEST));
         }
     } else if (client->mode == MODE_COMMAND) {
         buffer_appendf(buffer, buf_size, pos,
diff --git a/tests/test_anonymous_access.sh b/tests/test_anonymous_access.sh
index 8930049..f325d0b 100755
--- a/tests/test_anonymous_access.sh
+++ b/tests/test_anonymous_access.sh
@@ -10,7 +10,7 @@ if [ ! -f "$BIN" ]; then
 fi
 
 echo "Starting TNT server on port $PORT..."
-$BIN -p $PORT > /dev/null 2>&1 &
+TNT_LANG=zh $BIN -p $PORT > /dev/null 2>&1 &
 SERVER_PID=$!
 sleep 2
 
diff --git a/tests/test_connection_limits.sh b/tests/test_connection_limits.sh
index fa1deec..d0336dc 100755
--- a/tests/test_connection_limits.sh
+++ b/tests/test_connection_limits.sh
@@ -44,7 +44,7 @@ wait_for_health() {
 
 echo "=== TNT Connection Limit Tests ==="
 
-TNT_RATE_LIMIT=0 TNT_MAX_CONN_PER_IP=1 "$BIN" -p "$PORT" -d "$STATE_DIR" \
+TNT_LANG=zh TNT_RATE_LIMIT=0 TNT_MAX_CONN_PER_IP=1 "$BIN" -p "$PORT" -d "$STATE_DIR" \
     >"$STATE_DIR/concurrent.log" 2>&1 &
 SERVER_PID=$!
 
@@ -99,7 +99,7 @@ SERVER_PID=""
 RATE_PORT=$((PORT + 1))
 SSH_RATE_OPTS="-o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o BatchMode=yes -p $RATE_PORT"
 
-TNT_MAX_CONN_PER_IP=10 TNT_MAX_CONN_RATE_PER_IP=2 "$BIN" -p "$RATE_PORT" -d "$STATE_DIR" \
+TNT_LANG=zh TNT_MAX_CONN_PER_IP=10 TNT_MAX_CONN_RATE_PER_IP=2 "$BIN" -p "$RATE_PORT" -d "$STATE_DIR" \
     >"$STATE_DIR/rate.log" 2>&1 &
 SERVER_PID=$!
 
diff --git a/tests/test_exec_mode.sh b/tests/test_exec_mode.sh
index a03d288..3974dff 100755
--- a/tests/test_exec_mode.sh
+++ b/tests/test_exec_mode.sh
@@ -29,7 +29,7 @@ SSH_OPTS="-n -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o Batc
 
 echo "=== TNT Exec Mode Tests ==="
 
-TNT_RATE_LIMIT=0 $BIN -p "$PORT" -d "$STATE_DIR" >"${STATE_DIR}/server.log" 2>&1 &
+TNT_LANG=zh TNT_RATE_LIMIT=0 $BIN -p "$PORT" -d "$STATE_DIR" >"${STATE_DIR}/server.log" 2>&1 &
 SERVER_PID=$!
 
 HEALTH_OUTPUT=""
@@ -76,8 +76,8 @@ else
 fi
 
 SUPPORT_OUTPUT=$(ssh $SSH_OPTS localhost support 2>/dev/null || true)
-printf '%s\n' "$SUPPORT_OUTPUT" | grep -q '^TNT support$' &&
-printf '%s\n' "$SUPPORT_OUTPUT" | grep -q '^Troubleshooting:'
+printf '%s\n' "$SUPPORT_OUTPUT" | grep -Eq '^TNT (support|支持)$' &&
+printf '%s\n' "$SUPPORT_OUTPUT" | grep -Eq '^(Troubleshooting|排查):'
 if [ $? -eq 0 ]; then
     echo "✓ support returns quick guide"
     PASS=$((PASS + 1))
diff --git a/tests/test_interactive_input.sh b/tests/test_interactive_input.sh
index bc5cc2a..89a485f 100755
--- a/tests/test_interactive_input.sh
+++ b/tests/test_interactive_input.sh
@@ -32,7 +32,7 @@ SSH_OPTS="-e none -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o
 
 echo "=== TNT Interactive Input Tests ==="
 
-TNT_RATE_LIMIT=0 "$BIN" -p "$PORT" -d "$STATE_DIR" >"$STATE_DIR/server.log" 2>&1 &
+TNT_LANG=zh TNT_RATE_LIMIT=0 "$BIN" -p "$PORT" -d "$STATE_DIR" >"$STATE_DIR/server.log" 2>&1 &
 SERVER_PID=$!
 
 SERVER_READY=0
diff --git a/tests/unit/Makefile b/tests/unit/Makefile
index 333db75..c00be97 100644
--- a/tests/unit/Makefile
+++ b/tests/unit/Makefile
@@ -15,8 +15,9 @@ MESSAGE_SRC = ../../src/message.c
 COMMON_SRC = ../../src/common.c
 CHAT_ROOM_SRC = ../../src/chat_room.c
 HISTORY_VIEW_SRC = ../../src/history_view.c
+I18N_SRC = ../../src/i18n.c
 
-TESTS = test_utf8 test_message test_chat_room test_history_view
+TESTS = test_utf8 test_message test_chat_room test_history_view test_i18n
 
 .PHONY: all clean run
 
@@ -34,6 +35,9 @@ test_chat_room: test_chat_room.c $(CHAT_ROOM_SRC) $(MESSAGE_SRC) $(UTF8_SRC) $(C
 test_history_view: test_history_view.c $(HISTORY_VIEW_SRC)
 	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
 
+test_i18n: test_i18n.c $(I18N_SRC)
+	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+
 run: all
 	@echo "=== Running UTF-8 Tests ==="
 	./test_utf8
@@ -46,6 +50,9 @@ run: all
 	@echo ""
 	@echo "=== Running History View Tests ==="
 	./test_history_view
+	@echo ""
+	@echo "=== Running i18n Tests ==="
+	./test_i18n
 
 clean:
 	rm -f $(TESTS) *.o test_messages.log
diff --git a/tests/unit/test_i18n.c b/tests/unit/test_i18n.c
new file mode 100644
index 0000000..c6a1bee
--- /dev/null
+++ b/tests/unit/test_i18n.c
@@ -0,0 +1,72 @@
+/* Unit tests for i18n language selection and text lookup */
+
+#include "../../include/i18n.h"
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define TEST(name) static void test_##name()
+#define RUN_TEST(name) do { \
+    printf("Running %s... ", #name); \
+    test_##name(); \
+    printf("✓\n"); \
+    tests_passed++; \
+} while(0)
+
+static int tests_passed = 0;
+
+TEST(parse_explicit_languages) {
+    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("cn", 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);
+}
+
+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);
+}
+
+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);
+
+    setenv("TNT_LANG", "en", 1);
+    setenv("LC_ALL", "zh_CN.UTF-8", 1);
+    assert(i18n_default_lang() == 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);
+
+    setenv("LC_ALL", "C", 1);
+    assert(i18n_default_lang() == LANG_EN);
+}
+
+TEST(text_lookup_matches_language) {
+    assert(strstr(i18n_text(LANG_EN, I18N_USERNAME_PROMPT),
+                  "display name") != NULL);
+    assert(strstr(i18n_text(LANG_ZH, I18N_USERNAME_PROMPT),
+                  "用户名") != NULL);
+    assert(strcmp(i18n_lang_code(LANG_EN), "en") == 0);
+    assert(strcmp(i18n_lang_code(LANG_ZH), "zh") == 0);
+}
+
+int main(void) {
+    printf("Running i18n unit tests...\n\n");
+
+    RUN_TEST(parse_explicit_languages);
+    RUN_TEST(parse_unknown_uses_fallback);
+    RUN_TEST(default_prefers_tnt_lang);
+    RUN_TEST(default_uses_locale_when_no_tnt_lang);
+    RUN_TEST(text_lookup_matches_language);
+
+    printf("\n✓ All %d tests passed!\n", tests_passed);
+    return 0;
+}
diff --git a/tnt.1 b/tnt.1
index 5ddd6c4..7ca1db9 100644
--- a/tnt.1
+++ b/tnt.1
@@ -156,6 +156,14 @@ Directory for host key and message log (default: current directory).
 If set, clients must supply this string as their SSH password.
 Compared in constant time.
 .TP
+.B TNT_LANG
+Default interactive UI language.
+Accepts
+.B en
+or
+.BR zh .
+When unset, TNT detects the process locale and falls back to English.
+.TP
 .B TNT_MAX_CONNECTIONS
 Global connection limit (default: 64, max: 1024).
 .TP