refactor(app): trust the server's single_language_mode, drop the client mirror (after #10097)

app/lib/backend/http/api/users.dart

@@ -339,18 +339,22 @@ Future<String?> getUserPrimaryLanguage() async {
   }
 }
 
-Future<bool> setUserPrimaryLanguage(String languageCode) async {
+/// Returns the server-decided `single_language_mode` on success, null on
+/// failure. The server derives eligibility from the live STT capability
+/// policy (#10022) — clients must not re-decide it locally.
+Future<bool?> setUserPrimaryLanguage(String languageCode) async {
   var response = await makeApiCall(
     url: '${Env.apiBaseUrl}v1/users/language',
     headers: {},
     method: 'PATCH',
     body: jsonEncode({'language': languageCode}),
   );
-  if (response == null) return false;
+  if (response == null) return null;
   Logger.debug('setUserPrimaryLanguage response: ${response.body}');
-  if (response.statusCode != 200) return false;
+  if (response.statusCode != 200) return null;
   final data = wire.GeneratedUserLanguageUpdateResponse.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
-  return data.status == 'ok';
+  if (data.status != 'ok') return null;
+  return data.singleLanguageMode;
 }
 
 Future<bool> setPreferredSummarizationAppServer(String appId) async {

app/lib/pages/onboarding/primary_language/primary_language_widget.dart

@@ -353,7 +353,21 @@ class _PrimaryLanguageWidgetState extends State<PrimaryLanguageWidget> {
                             // Update the user's primary language
                             final homeProvider = Provider.of<HomeProvider>(context, listen: false);
                             final userProvider = Provider.of<UserProvider>(context, listen: false);
-                            await homeProvider.updateUserPrimaryLanguage(selectedLanguage!, userProvider: userProvider);
+                            final success = await homeProvider.updateUserPrimaryLanguage(
+                              selectedLanguage!,
+                              userProvider: userProvider,
+                            );
+
+                            if (!context.mounted) return;
+                            if (!success) {
+                              ScaffoldMessenger.of(context).showSnackBar(
+                                SnackBar(
+                                  content: Text(context.l10n.failedToSetLanguage),
+                                  backgroundColor: Colors.red,
+                                ),
+                              );
+                              return;
+                            }
 
                             widget.goNext();
                           },

app/lib/providers/home_provider.dart

@@ -9,81 +9,6 @@ import 'package:omi/pages/settings/language_selection_dialog.dart';
 import 'package:omi/providers/user_provider.dart';
 import 'package:omi/utils/logger.dart';
 
-/// Base language codes eligible for live multi-language auto-detection.
-/// Mirrors the backend live STT capability policy
-/// (backend/config/stt_provider_policy.py MODULATE_SUPPORTED_LANGUAGES, #10022);
-/// regional variants ("pt-BR") are normalized to their base code before lookup.
-const multiLanguageSupported = {
-  'multi',
-  'af',
-  'ar',
-  'az',
-  'be',
-  'bg',
-  'bn',
-  'bs',
-  'ca',
-  'cs',
-  'cy',
-  'da',
-  'de',
-  'el',
-  'en',
-  'es',
-  'et',
-  'eu',
-  'fa',
-  'fi',
-  'fr',
-  'gl',
-  'gu',
-  'he',
-  'hi',
-  'hr',
-  'hu',
-  'id',
-  'it',
-  'ja',
-  'kk',
-  'kn',
-  'ko',
-  'lt',
-  'lv',
-  'mk',
-  'ml',
-  'mr',
-  'ms',
-  'nl',
-  'no',
-  'pa',
-  'pl',
-  'pt',
-  'ro',
-  'ru',
-  'sk',
-  'sl',
-  'sq',
-  'sr',
-  'sv',
-  'sw',
-  'ta',
-  'te',
-  'th',
-  'tl',
-  'tr',
-  'uk',
-  'ur',
-  'vi',
-  'zh',
-};
-
-/// Whether a (possibly regional) language code may enter live multi-language
-/// mode, matching the backend's normalized policy check.
-bool supportsLiveMultilingualMode(String languageCode) {
-  final base = languageCode.split('-').first.split('_').first.toLowerCase();
-  return multiLanguageSupported.contains(base);
-}
-
 class HomeProvider extends ChangeNotifier {
   int _sessionGeneration = 0;
   int selectedIndex = 0;
@@ -316,17 +241,17 @@ class HomeProvider extends ChangeNotifier {
 
   Future<bool> updateUserPrimaryLanguage(String languageCode, {UserProvider? userProvider}) async {
     try {
-      final success = await setUserPrimaryLanguage(languageCode);
-      if (success) {
+      final serverSingleLanguageMode = await setUserPrimaryLanguage(languageCode);
+      if (serverSingleLanguageMode != null) {
         userPrimaryLanguage = languageCode;
         hasSetPrimaryLanguage = true;
         SharedPreferencesUtil().userPrimaryLanguage = languageCode;
         SharedPreferencesUtil().hasSetPrimaryLanguage = true;
         PlatformManager.instance.analytics.setUserAttribute('Primary Language', languageCode);
 
-        // Backend auto-sets single_language_mode — sync local state to match
-        final singleLanguageMode = !supportsLiveMultilingualMode(languageCode);
-        userProvider?.updateSingleLanguageModeLocally(singleLanguageMode);
+        // The server decides single_language_mode from the live STT policy
+        // (#10022); local state mirrors its response, never a client-side list.
+        userProvider?.updateSingleLanguageModeLocally(serverSingleLanguageMode);
 
         notifyListeners();
         return true;

app/test/unit/language_update_response_test.dart

@@ -0,0 +1,29 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:omi/backend/schema/gen/users_wire.g.dart';
+
+/// #10022 follow-up: the client no longer mirrors the backend's multi-language
+/// eligibility list — it trusts the server's `single_language_mode` in the
+/// PATCH /v1/users/language response. This pins the wire contract that makes
+/// that trust safe.
+void main() {
+  test('language update response carries the server-decided mode', () {
+    final multilingual = GeneratedUserLanguageUpdateResponse.fromJson(
+      {'status': 'ok', 'single_language_mode': false},
+    );
+    expect(multilingual.status, 'ok');
+    expect(multilingual.singleLanguageMode, isFalse);
+
+    final single = GeneratedUserLanguageUpdateResponse.fromJson(
+      {'status': 'ok', 'single_language_mode': true},
+    );
+    expect(single.singleLanguageMode, isTrue);
+  });
+
+  test('a response without the mode field fails loudly instead of defaulting', () {
+    expect(
+      () => GeneratedUserLanguageUpdateResponse.fromJson({'status': 'ok'}),
+      throwsA(isA<Object>()),
+    );
+  });
+}