added: caption language stem utility to support language variant conversion between videos.

Author: karyogamy

app/src/main/java/org/schabi/newpipe/player/Player.java

@@ -46,6 +46,7 @@
 import static org.schabi.newpipe.player.helper.PlayerHelper.MinimizeMode.MINIMIZE_ON_EXIT_MODE_NONE;
 import static org.schabi.newpipe.player.helper.PlayerHelper.MinimizeMode.MINIMIZE_ON_EXIT_MODE_POPUP;
 import static org.schabi.newpipe.player.helper.PlayerHelper.buildCloseOverlayLayoutParams;
+import static org.schabi.newpipe.player.helper.PlayerHelper.captionLanguageStemOf;
 import static org.schabi.newpipe.player.helper.PlayerHelper.formatSpeed;
 import static org.schabi.newpipe.player.helper.PlayerHelper.getMinimizeOnExitAction;
 import static org.schabi.newpipe.player.helper.PlayerHelper.getMinimumVideoHeight;
@@ -3542,7 +3543,8 @@ private void buildCaptionMenu(@NonNull final List<String> availableLanguages) {
                     // if a track of userPreferredLanguage is not found
                     // This means (auto-generated) will be resolved automatically.
                     trackSelector.setParameters(trackSelector.buildUponParameters()
-                            .setPreferredTextLanguage(captionLanguage)
+                            .setPreferredTextLanguages(captionLanguage,
+                                    captionLanguageStemOf(captionLanguage))
                             .setRendererDisabled(textRendererIndex, false));
                     prefs.edit().putString(context.getString(R.string.caption_user_set_key),
                             captionLanguage).apply();
@@ -3563,7 +3565,8 @@ private void buildCaptionMenu(@NonNull final List<String> availableLanguages) {
                 && !selectedPreferredLanguages.contains(userPreferredLanguage)
                 && textRendererIndex != RENDERER_UNAVAILABLE) {
             trackSelector.setParameters(trackSelector.buildUponParameters()
-                    .setPreferredTextLanguage(userPreferredLanguage)
+                    .setPreferredTextLanguages(userPreferredLanguage,
+                            captionLanguageStemOf(userPreferredLanguage))
                     .setRendererDisabled(textRendererIndex, false));
         }
     }

app/src/main/java/org/schabi/newpipe/player/helper/PlayerHelper.java

@@ -144,6 +144,21 @@ public static String captionLanguageOf(@NonNull final Context context,
                 ? " (" + context.getString(R.string.caption_auto_generated) + ")" : "");
     }
 
+    @NonNull
+    public static String captionLanguageStemOf(@NonNull final String language) {
+        if (!language.contains("(") || !language.contains(")")) {
+            return language;
+        }
+
+        if (language.startsWith("(")) {
+            // language text is right-to-left
+            final String[] parts = language.split("\\)");
+            return parts[parts.length - 1].trim();
+        }
+
+        return language.split("\\(")[0].trim();
+    }
+
     @NonNull
     public static String resizeTypeOf(@NonNull final Context context,
                                       @ResizeMode final int resizeMode) {