app: Override locale with language global setting

This doesn't really have user-facing changes yet, because they cannot
set the language.  In this case, ZulipApp.locale is null, and
localization will follow system setting, like we did before this change.

This behavior is an improvement compared to the legacy app, which just uses
English (en) when language is not set.

Author: PIG208

lib/widgets/app.dart

@@ -223,6 +223,7 @@ class _ZulipAppState extends State<ZulipApp> with WidgetsBindingObserver {
           onGenerateTitle: (BuildContext context) {
             return ZulipLocalizations.of(context).zulipAppTitle;
           },
+          locale: GlobalStoreWidget.settingsOf(context).language,
           localizationsDelegates: ZulipLocalizations.localizationsDelegates,
           supportedLocales: ZulipLocalizations.supportedLocales,
           // The context has to be taken from the [Builder] because

test/widgets/app_test.dart

@@ -23,6 +23,37 @@ import 'test_app.dart';
 void main() {
   TestZulipBinding.ensureInitialized();
 
+  group('ZulipApp locale', () {
+    Future<void> prepare(WidgetTester tester, {required Locale? locale}) async {
+      addTearDown(testBinding.reset);
+      await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
+      if (locale != null) {
+        await testBinding.globalStore.settings.setLanguage(locale);
+      }
+      await tester.pumpWidget(ZulipApp());
+      await tester.pump();
+      await tester.pump();
+    }
+
+    testWidgets('no language is set', (tester) async {
+      await prepare(tester, locale: null);
+      final element = tester.element(find.byType(HomePage));
+      check(Localizations.localeOf(element)).equals(const Locale('en'));
+    });
+
+    testWidgets('has language set', (tester) async {
+      await prepare(tester, locale: const Locale('uk'));
+      final element = tester.element(find.byType(HomePage));
+      check(Localizations.localeOf(element)).equals(const Locale('uk'));
+    });
+
+    testWidgets('has unsupported language set', (tester) async {
+      await prepare(tester, locale: const Locale('zxx'));
+      final element = tester.element(find.byType(HomePage));
+      check(Localizations.localeOf(element)).equals(const Locale('en'));
+    });
+  });
+
   group('ZulipApp initial navigation', () {
     late List<Route<dynamic>> pushedRoutes = [];
 

test/widgets/test_app.dart

@@ -71,6 +71,7 @@ class TestZulipApp extends StatelessWidget {
 
       return MaterialApp(
         title: 'Zulip',
+        locale: GlobalStoreWidget.settingsOf(context).language,
         localizationsDelegates: ZulipLocalizations.localizationsDelegates,
         supportedLocales: ZulipLocalizations.supportedLocales,
         // The context has to be taken from the [Builder] because