fix(metadata): send Accept-Language on Deezer requests so names follow the app language

Deezer localizes artist and genre names by IP geolocation when no language
is given, so users behind e.g. Arabic-region IPs got Arabic metadata on an
English device (VPN 'fixed' it). The app's display language is now pushed
to the backend and sent as Accept-Language, with an en-US default.

Fixes #480

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivity.kt

@@ -3152,6 +3152,13 @@ class MainActivity: FlutterFragmentActivity() {
                             }
                             result.success(null)
                         }
+                        "setMetadataLanguage" -> {
+                            val tag = call.argument<String>("tag") ?: ""
+                            withContext(Dispatchers.IO) {
+                                Gobackend.setMetadataLanguage(tag)
+                            }
+                            result.success(null)
+                        }
                         "getLogCount" -> {
                             val count = withContext(Dispatchers.IO) {
                                 Gobackend.getLogCount()

go_backend/deezer.go

@@ -1321,6 +1321,10 @@ func (c *DeezerClient) doGetJSON(ctx context.Context, endpoint string, dst any)
 	}
 
 	req.Header.Set("Accept", "application/json")
+	// Without an explicit language Deezer localizes artist/genre names by
+	// the caller's IP geolocation (issue #480: Arabic metadata on an
+	// English device). Follow the app's display language instead.
+	req.Header.Set("Accept-Language", metadataAcceptLanguage())
 
 	resp, err := c.httpClient.Do(req)
 	if err != nil {

go_backend/exports.go

@@ -4,8 +4,33 @@ import (
 	"encoding/json"
 	"runtime/debug"
 	"strings"
+	"sync"
 )
 
+var (
+	metadataLanguageMu  sync.RWMutex
+	metadataLanguageTag string
+)
+
+// SetMetadataLanguage sets the app's display language (BCP 47 tag, e.g.
+// "en-US" or "id"), used as Accept-Language on metadata API requests so
+// providers localize names by the app language instead of IP geolocation.
+func SetMetadataLanguage(tag string) {
+	metadataLanguageMu.Lock()
+	metadataLanguageTag = strings.TrimSpace(tag)
+	metadataLanguageMu.Unlock()
+}
+
+func metadataAcceptLanguage() string {
+	metadataLanguageMu.RLock()
+	tag := metadataLanguageTag
+	metadataLanguageMu.RUnlock()
+	if tag == "" || strings.HasPrefix(strings.ToLower(tag), "en") {
+		return "en-US,en;q=0.9"
+	}
+	return tag + ",en;q=0.8"
+}
+
 // ReleaseMemory drops idle pooled extension runtimes, forces a GC, and
 // returns freed heap to the OS. Called from the app on OS memory pressure and
 // when backgrounded, so the Go side's RSS doesn't sit at its high-water mark

lib/screens/main_shell.dart

@@ -75,6 +75,13 @@ class _MainShellState extends ConsumerState<MainShell>
     setPlaybackNormalizationEnabled(
       ref.read(settingsProvider).playbackNormalization,
     );
+    // Deezer & co. localize artist/genre names by IP unless told the app's
+    // language (issue #480).
+    unawaited(
+      PlatformBridge.setMetadataLanguage(
+        Localizations.localeOf(context).toLanguageTag(),
+      ),
+    );
   }
 
   @override

lib/services/platform_bridge.dart

@@ -1210,6 +1210,14 @@ class PlatformBridge {
     } catch (_) {}
   }
 
+  /// Tells the backend the app's display language so metadata providers
+  /// localize by it instead of IP geolocation. Best-effort.
+  static Future<void> setMetadataLanguage(String tag) async {
+    try {
+      await _channel.invokeMethod('setMetadataLanguage', {'tag': tag});
+    } catch (_) {}
+  }
+
   static Future<int> getGoLogCount() async {
     final result = await _channel.invokeMethod('getLogCount');
     return result as int;