Add language setting

assets/l10n/app_en.arb

@@ -883,5 +883,13 @@
   "zulipAppTitle": "Zulip",
   "@zulipAppTitle": {
     "description": "The name of Zulip. This should be either 'Zulip' or a transliteration."
+  },
+  "languageSettingsTitle": "Language",
+  "@languageSettingsTitle": {
+    "description": "Title for the language settings section in the app settings."
+  },
+  "systemDefaultLanguage": "System default",
+  "@systemDefaultLanguage": {
+    "description": "Option to use the system default language setting."
   }
 }

lib/generated/l10n/zulip_localizations.dart

@@ -1303,6 +1303,18 @@ abstract class ZulipLocalizations {
   /// In en, this message translates to:
   /// **'Zulip'**
   String get zulipAppTitle;
+
+  /// Title for the language settings section in the app settings.
+  ///
+  /// In en, this message translates to:
+  /// **'Language'**
+  String get languageSettingsTitle;
+
+  /// Option to use the system default language setting.
+  ///
+  /// In en, this message translates to:
+  /// **'System default'**
+  String get systemDefaultLanguage;
 }
 
 class _ZulipLocalizationsDelegate

lib/generated/l10n/zulip_localizations_ar.dart

@@ -717,4 +717,10 @@ class ZulipLocalizationsAr extends ZulipLocalizations {
 
   @override
   String get zulipAppTitle => 'Zulip';
+
+  @override
+  String get languageSettingsTitle => 'Language';
+
+  @override
+  String get systemDefaultLanguage => 'System default';
 }

lib/generated/l10n/zulip_localizations_en.dart

@@ -717,4 +717,10 @@ class ZulipLocalizationsEn extends ZulipLocalizations {
 
   @override
   String get zulipAppTitle => 'Zulip';
+
+  @override
+  String get languageSettingsTitle => 'Language';
+
+  @override
+  String get systemDefaultLanguage => 'System default';
 }

lib/generated/l10n/zulip_localizations_ja.dart

@@ -717,4 +717,10 @@ class ZulipLocalizationsJa extends ZulipLocalizations {
 
   @override
   String get zulipAppTitle => 'Zulip';
+
+  @override
+  String get languageSettingsTitle => 'Language';
+
+  @override
+  String get systemDefaultLanguage => 'System default';
 }

lib/generated/l10n/zulip_localizations_nb.dart

@@ -717,4 +717,10 @@ class ZulipLocalizationsNb extends ZulipLocalizations {
 
   @override
   String get zulipAppTitle => 'Zulip';
+
+  @override
+  String get languageSettingsTitle => 'Language';
+
+  @override
+  String get systemDefaultLanguage => 'System default';
 }

lib/generated/l10n/zulip_localizations_pl.dart

@@ -728,4 +728,10 @@ class ZulipLocalizationsPl extends ZulipLocalizations {
 
   @override
   String get zulipAppTitle => 'Zulip';
+
+  @override
+  String get languageSettingsTitle => 'Language';
+
+  @override
+  String get systemDefaultLanguage => 'System default';
 }

lib/generated/l10n/zulip_localizations_ru.dart

@@ -731,4 +731,10 @@ class ZulipLocalizationsRu extends ZulipLocalizations {
 
   @override
   String get zulipAppTitle => 'Zulip';
+
+  @override
+  String get languageSettingsTitle => 'Language';
+
+  @override
+  String get systemDefaultLanguage => 'System default';
 }

lib/generated/l10n/zulip_localizations_sk.dart

@@ -719,4 +719,10 @@ class ZulipLocalizationsSk extends ZulipLocalizations {
 
   @override
   String get zulipAppTitle => 'Zulip';
+
+  @override
+  String get languageSettingsTitle => 'Language';
+
+  @override
+  String get systemDefaultLanguage => 'System default';
 }

lib/generated/l10n/zulip_localizations_uk.dart

@@ -731,4 +731,10 @@ class ZulipLocalizationsUk extends ZulipLocalizations {
 
   @override
   String get zulipAppTitle => 'Zulip';
+
+  @override
+  String get languageSettingsTitle => 'Language';
+
+  @override
+  String get systemDefaultLanguage => 'System default';
 }

lib/widgets/app.dart

@@ -1,8 +1,11 @@
 import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart';
+import 'package:path_provider/path_provider.dart';
 
 import '../generated/l10n/zulip_localizations.dart';
 import '../log.dart';
@@ -47,6 +50,63 @@ class ZulipApp extends StatefulWidget {
     return completer.future;
   }
 
+  // static Locale? currentLocale;
+  // static final ValueNotifier<Locale?> _localeNotifier = ValueNotifier(currentLocale);
+  //
+  // static void setLocale(Locale? locale) {
+  //   currentLocale = locale;
+  //   _localeNotifier.value = locale;
+  // }
+
+  static Locale? currentLocale;
+  static final ValueNotifier<Locale?> _localeNotifier = ValueNotifier(currentLocale);
+  static File? _prefsFile;
+
+  static Future<File> _getPrefsFile() async {
+    if (_prefsFile == null) {
+      final dir = await getApplicationDocumentsDirectory();
+      _prefsFile = File('${dir.path}/language_prefs.json');
+      if (!await _prefsFile!.exists()) {
+        await _prefsFile!.create();
+      }
+    }
+    return _prefsFile!;
+  }
+
+  static Future<void> setLocale(Locale? locale) async {
+    currentLocale = locale;
+    _localeNotifier.value = locale;
+
+    try {
+      final file = await _getPrefsFile();
+      await file.writeAsString(
+        jsonEncode({'locale': locale?.toString()}),
+        flush: true,
+      );
+    } catch (e) {
+      debugPrint('Error saving locale: $e');
+    }
+  }
+
+  static Future<void> loadSavedLocale() async {
+    try {
+      final file = await _getPrefsFile();
+      final content = await file.readAsString();
+      if (content.isNotEmpty) {
+        final data = jsonDecode(content) as Map<String, dynamic>;
+        final localeString = data['locale'] as String?;
+        if (localeString != null && localeString.isNotEmpty) {
+          final parts = localeString.split('_');
+          currentLocale = parts.length == 2
+              ? Locale(parts[0], parts[1])
+              : Locale(parts[0]);
+          _localeNotifier.value = currentLocale;
+        }
+      }
+    } catch (e) {
+      debugPrint('Error loading locale: $e');
+    }
+  }
   /// A key for the navigator for the whole app.
   ///
   /// For code that exists entirely outside the widget tree and has no natural
@@ -160,14 +220,26 @@ class _ZulipAppState extends State<ZulipApp> with WidgetsBindingObserver {
   void initState() {
     super.initState();
     WidgetsBinding.instance.addObserver(this);
+    ZulipApp._localeNotifier.addListener(_handleLocaleChange);
+    _initializeApp();
+  }
+
+  Future<void> _initializeApp() async {
+    await ZulipApp.loadSavedLocale(); // Load saved locale first
+    ZulipApp._localeNotifier.addListener(_handleLocaleChange);
   }
 
   @override
   void dispose() {
+    ZulipApp._localeNotifier.removeListener(_handleLocaleChange);
     WidgetsBinding.instance.removeObserver(this);
     super.dispose();
   }
 
+  void _handleLocaleChange() {
+    if (mounted) setState(() {});
+  }
+
   List<Route<dynamic>> _handleGenerateInitialRoutes(String initialRoute) {
     // The `_ZulipAppState.context` lacks the required ancestors. Instead
     // we use the Navigator which should be available when this callback is
@@ -220,6 +292,7 @@ class _ZulipAppState extends State<ZulipApp> with WidgetsBindingObserver {
     return GlobalStoreWidget(
       child: Builder(builder: (context) {
         return MaterialApp(
+          locale: ZulipApp.currentLocale,
           onGenerateTitle: (BuildContext context) {
             return ZulipLocalizations.of(context).zulipAppTitle;
           },