Feature: Add support for localization, introduce slang for translations, and integrate German and English locale support throughout the app

finlog_app/app/lib/core/ui/controller/locale_controller.dart

@@ -1,44 +1,74 @@
+import 'package:app/core/i18n/translations.g.dart';
 import 'package:flutter/material.dart';
 import 'package:fluttery/fluttery.dart';
 import 'package:fluttery/preferences.dart';
 
 class LocaleController extends ChangeNotifier {
-  final Preferences _prefs;
-
-  LocaleController() : _prefs = App.service<Preferences>();
+  final Preferences _prefs = App.service<Preferences>();
 
   static const _key = 'language';
+  LanguagePref _current = LanguagePref.system;
 
-  LanguagePref _current = LanguagePref.en; // Default = Englisch
-
+  /// Einmal beim App-Start aufrufen
   Future<void> init() async {
     final saved = await _prefs.getString(_key);
-    _current = switch (saved) {
-      'de' => LanguagePref.de,
-      'system' => LanguagePref.system,
-      'en' || _ => LanguagePref.en, // default fallback = en
-    };
+    _current = _fromString(saved) ?? LanguagePref.system;
+
+    _applyToSlang(_current);
     notifyListeners();
   }
 
-  void setLanguage(LanguagePref pref) {
-    if (_current == pref) return;
-    _current = pref;
+  /// Sprache ändern (persistieren + sofort anwenden)
+  Future<void> setLanguage(LanguagePref lang) async {
+    _current = lang;
+    await _prefs.setString(_key, lang.name);
+
+    _applyToSlang(lang);
     notifyListeners();
-    _prefs.setString(_key, pref.name); // fire-and-forget
+  }
+
+  void _applyToSlang(LanguagePref pref) {
+    if (pref == LanguagePref.system) {
+      LocaleSettings.useDeviceLocale();
+      return;
+    }
+
+    final code = pref.code;
+    if (code == null) {
+      LocaleSettings.useDeviceLocale();
+      return;
+    }
+
+    if (AppLocaleUtils.supportedLocalesRaw.contains(code)) {
+      LocaleSettings.setLocaleRaw(code);
+    } else {
+      LocaleSettings.setLocale(AppLocale.en);
+    }
+  }
+
+  LanguagePref? _fromString(String? value) {
+    if (value == null || value.isEmpty) return null;
+    return LanguagePref.values.firstWhere(
+      (e) => e.name == value,
+      orElse: () => LanguagePref.system,
+    );
   }
 
   LanguagePref get language => _current;
 
   Locale? get locale => _current.locale;
+
+  bool get isSystem => _current == LanguagePref.system;
 }
 
 enum LanguagePref {
-  system(null), // folgt System
-  de(Locale('de')), // Deutsch
-  en(Locale('en')); // Englisch (default)
+  system(null),
+  de(Locale('de')),
+  en(Locale('en'));
 
   final Locale? locale;
 
   const LanguagePref(this.locale);
+
+  String? get code => locale?.languageCode;
 }