Add language suffix to downloaded subtitle filenames

app/src/main/java/org/schabi/newpipe/download/DownloadDialog.java

@@ -75,6 +75,7 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Objects;
+import java.util.Optional;
 
 import icepick.Icepick;
 import icepick.State;
@@ -562,6 +563,39 @@ public void onItemSelected(final AdapterView<?> parent,
                 selectedSubtitleIndex = position;
                 break;
         }
+        onItemSelectedSetFileName();
+    }
+
+    private void onItemSelectedSetFileName() {
+        final String fileName = FilenameUtils.createFilename(getContext(), currentInfo.getName());
+        final String prevFileName = Optional.ofNullable(dialogBinding.fileName.getText())
+                .map(Object::toString)
+                .orElse("");
+
+        if (prevFileName.isEmpty()
+                || prevFileName.equals(fileName)
+                || prevFileName.startsWith(getString(R.string.caption_file_name, fileName, ""))) {
+            // only update the file name field if it was not edited by the user
+
+            switch (dialogBinding.videoAudioGroup.getCheckedRadioButtonId()) {
+                case R.id.audio_button:
+                case R.id.video_button:
+                    if (!prevFileName.equals(fileName)) {
+                        // since the user might have switched between audio and video, the correct
+                        // text might already be in place, so avoid resetting the cursor position
+                        dialogBinding.fileName.setText(fileName);
+                    }
+                    break;
+
+                case R.id.subtitle_button:
+                    final String setSubtitleLanguageCode = subtitleStreamsAdapter
+                            .getItem(selectedSubtitleIndex).getLanguageTag();
+                    // this will reset the cursor position, which is bad UX, but it can't be avoided
+                    dialogBinding.fileName.setText(getString(
+                            R.string.caption_file_name, fileName, setSubtitleLanguageCode));
+                    break;
+            }
+        }
     }
 
     @Override

app/src/main/res/values/donottranslate.xml

@@ -27,4 +27,5 @@
     <string name="soundcloud">SoundCloud</string>
     <string name="preferred_open_action_share_menu_title">@string/app_name</string>
     <string name="leakcanary">LeakCanary</string>
+    <string name="caption_file_name">%1$s-%2$s</string>
 </resources>