#256 finalize translation service

lib/screens/app_settings_screen.dart

@@ -1387,7 +1387,7 @@ class _TranslationLanguageDialogContent extends StatefulWidget {
   });
 
   final String? currentLanguageCode;
-  final ValueChanged<String> onLanguageSelected;
+  final ValueChanged<String?> onLanguageSelected;
 
   @override
   State<_TranslationLanguageDialogContent> createState() =>
@@ -1438,17 +1438,20 @@ class _TranslationLanguageDialogContentState
             ),
             const SizedBox(height: 12),
             Flexible(
-              child: RadioGroup<String>(
+              child: RadioGroup<String?>(
                 groupValue: widget.currentLanguageCode,
                 onChanged: (value) {
-                  if (value == null) return;
                   widget.onLanguageSelected(value);
                 },
                 child: ListView(
                   shrinkWrap: true,
                   children: [
+                    RadioListTile<String?>(
+                      value: null,
+                      title: Text(context.l10n.translation_useAppLanguage),
+                    ),
                     for (final option in _filtered)
-                      RadioListTile<String>(
+                      RadioListTile<String?>(
                         value: option.code,
                         title: Text(option.label),
                         subtitle: Text(option.code.toUpperCase()),

lib/services/translation_service.dart

@@ -77,7 +77,10 @@ class TranslationService extends ChangeNotifier {
   }
 
   String? resolvedIncomingLanguageCode(String? fallbackLanguageCode) {
-    return _settings.languageOverride ?? fallbackLanguageCode ?? 'en';
+    return _settings.translationTargetLanguageCode ??
+        _settings.languageOverride ??
+        fallbackLanguageCode ??
+        'en';
   }
 
   bool shouldTranslateIncoming({

lib/widgets/message_translation_button.dart

@@ -36,7 +36,7 @@ Future<void> showMessageTranslationSheet({
   required bool enabled,
   required String? selectedLanguageCode,
   required ValueChanged<bool> onEnabledChanged,
-  required ValueChanged<String> onLanguageSelected,
+  required ValueChanged<String?> onLanguageSelected,
 }) {
   return showModalBottomSheet<void>(
     context: context,
@@ -54,7 +54,7 @@ class _MessageTranslationSheet extends StatefulWidget {
   final bool enabled;
   final String? selectedLanguageCode;
   final ValueChanged<bool> onEnabledChanged;
-  final ValueChanged<String> onLanguageSelected;
+  final ValueChanged<String?> onLanguageSelected;
 
   const _MessageTranslationSheet({
     required this.enabled,
@@ -145,9 +145,26 @@ class _MessageTranslationSheetState extends State<_MessageTranslationSheet> {
             Flexible(
               child: ListView.builder(
                 shrinkWrap: true,
-                itemCount: _filtered.length,
+                itemCount: _filtered.length + 1,
                 itemBuilder: (context, index) {
-                  final option = _filtered[index];
+                  if (index == 0) {
+                    final selected = _localSelectedLanguageCode == null;
+                    return ListTile(
+                      contentPadding: EdgeInsets.zero,
+                      leading: Icon(
+                        selected
+                            ? Icons.radio_button_checked
+                            : Icons.radio_button_unchecked,
+                      ),
+                      title: Text(context.l10n.translation_useAppLanguage),
+                      onTap: () {
+                        setState(() => _localSelectedLanguageCode = null);
+                        widget.onLanguageSelected(null);
+                        Navigator.pop(context);
+                      },
+                    );
+                  }
+                  final option = _filtered[index - 1];
                   final selected = option.code == _localSelectedLanguageCode;
                   return ListTile(
                     contentPadding: EdgeInsets.zero,