fix: preserve account language on telegram auth

backend/bot/app/web/webapp/auth.py

@@ -1203,7 +1203,7 @@ def _apply_telegram_profile_to_user(
     settings: Settings,
 ) -> None:
     language_code = _normalize_language(
-        telegram_user.get("language_code") or user.language_code or settings.DEFAULT_LANGUAGE
+        user.language_code or telegram_user.get("language_code") or settings.DEFAULT_LANGUAGE
     )
 
     user.telegram_id = int(telegram_user["id"])
@@ -1251,8 +1251,8 @@ async def _link_telegram_to_user(
 
     if not existing_telegram_user and int(current_user.user_id) < 0:
         language_code = _normalize_language(
-            telegram_user.get("language_code")
-            or current_user.language_code
+            current_user.language_code
+            or telegram_user.get("language_code")
             or settings.DEFAULT_LANGUAGE
         )
         target_user, _ = await user_dal.create_user(
@@ -1400,20 +1400,19 @@ async def _ensure_user_from_telegram(
     referral_param: Optional[str] = None,
 ) -> User:
     user_id = int(telegram_user["id"])
-    language_code = _normalize_language(
+    telegram_language_code = _normalize_language(
         telegram_user.get("language_code") or settings.DEFAULT_LANGUAGE
     )
 
-    update_data = {
+    profile_data = {
         "telegram_id": user_id,
         "username": sanitize_username(telegram_user.get("username")),
         "first_name": sanitize_display_name(telegram_user.get("first_name")),
         "last_name": sanitize_display_name(telegram_user.get("last_name")),
-        "language_code": language_code,
     }
     telegram_photo_url = _telegram_photo_url_value(telegram_user)
     if telegram_photo_url:
-        update_data["telegram_photo_url"] = telegram_photo_url
+        profile_data["telegram_photo_url"] = telegram_photo_url
 
     db_user = await user_dal.get_user_by_telegram_id(session, user_id)
     if not db_user:
@@ -1428,7 +1427,8 @@ async def _ensure_user_from_telegram(
             session,
             {
                 "user_id": user_id,
-                **update_data,
+                **profile_data,
+                "language_code": telegram_language_code,
                 "referred_by_id": referred_by_id,
                 "registration_date": datetime.now(timezone.utc),
             },
@@ -1436,6 +1436,10 @@ async def _ensure_user_from_telegram(
         setattr(db_user, "_webapp_created", bool(created))
         return db_user
 
+    update_data = {
+        **profile_data,
+        "language_code": _normalize_language(db_user.language_code or telegram_language_code),
+    }
     changed = {key: value for key, value in update_data.items() if getattr(db_user, key) != value}
     if changed:
         db_user = await user_dal.update_user(session, db_user.user_id, changed) or db_user

tests/test_account_linking_panel.py

@@ -7,6 +7,8 @@ from unittest.mock import AsyncMock, patch
 from bot.app.web import subscription_webapp  # noqa: F401
 from bot.app.web.webapp import account as account_routes
 from bot.app.web.webapp.auth import (
+    _apply_telegram_profile_to_user,
+    _ensure_user_from_telegram,
     _link_telegram_to_user,
     _panel_description_for_user,
     _sync_merged_panel_identity_for_user,
@@ -97,6 +99,70 @@ class AccountLinkingPanelTests(unittest.IsolatedAsyncioTestCase):
 
         self.assertEqual(_panel_description_for_user(user), "alice??\nSmith")
 
+    def test_telegram_profile_update_preserves_existing_language(self):
+        user = SimpleNamespace(
+            telegram_id=None,
+            username=None,
+            first_name=None,
+            last_name=None,
+            language_code="ru",
+            telegram_photo_url=None,
+        )
+
+        _apply_telegram_profile_to_user(
+            user,
+            {
+                "id": 42,
+                "username": "alice",
+                "first_name": "Alice",
+                "last_name": "",
+                "language_code": "en",
+            },
+            SimpleNamespace(DEFAULT_LANGUAGE="en"),
+        )
+
+        self.assertEqual(user.language_code, "ru")
+        self.assertEqual(user.username, "alice")
+
+    async def test_telegram_login_preserves_existing_language(self):
+        user = SimpleNamespace(
+            user_id=42,
+            telegram_id=42,
+            username="old",
+            first_name="Old",
+            last_name=None,
+            language_code="ru",
+            telegram_photo_url=None,
+        )
+        update_user = AsyncMock(return_value=user)
+
+        with (
+            patch(
+                "bot.app.web.webapp.auth.user_dal.get_user_by_telegram_id",
+                AsyncMock(return_value=user),
+            ),
+            patch(
+                "bot.app.web.webapp.auth.user_dal.get_user_by_id",
+                AsyncMock(return_value=None),
+            ),
+            patch("bot.app.web.webapp.auth.user_dal.update_user", update_user),
+        ):
+            result = await _ensure_user_from_telegram(
+                SimpleNamespace(),
+                {
+                    "id": 42,
+                    "username": "alice",
+                    "first_name": "Alice",
+                    "last_name": "",
+                    "language_code": "en",
+                },
+                SimpleNamespace(DEFAULT_LANGUAGE="en"),
+            )
+
+        self.assertIs(result, user)
+        changed = update_user.await_args.args[2]
+        self.assertNotEqual(changed.get("language_code"), "en")
+
     async def test_merged_panel_identity_deletes_source_before_updating_target(self):
         calls = []