feat: Add Spanish, Dutch, Russian, and Chinese languages

Adds support for Spanish, Dutch, Russian, and Chinese to the language selection menu. This includes adding the necessary translation strings and updating the app customization page to display these new language options.

lib/l10n/app_localizations_fr.dart

@@ -655,6 +655,18 @@ class AppLocalizationsFr extends AppLocalizations {
   @override
   String get italiano => 'Italien';
 
+  @override
+  String get espanol => 'Espagnol';
+
+  @override
+  String get nederlands => 'Néerlandais';
+
+  @override
+  String get russian => 'Russe';
+
+  @override
+  String get chinese => 'Chinois';
+
   @override
   String get deleteMessagesTitle => 'Supprimer les messages';