loadfile: compute audio lang for sub selection when using lavfi-complex

When using lavfi-complex, no single track populates current_track for audio.
We work around this by iterating over the list of tracks and using the first
non-null language from a selected track. If multiple tracks are selected
(i.e. used by the filter) and have conflicting language tags, we'll ignore
them all and maintain the previous behavior (null).

player/loadfile.c

@@ -579,6 +579,35 @@ static char **process_langs(char **in)
     return out;
 }
 
+const char *get_audio_lang(struct MPContext *mpctx)
+{
+    // If we have a single current audio track, this is simple.
+    if (mpctx->current_track[0][STREAM_AUDIO])
+        return mpctx->current_track[0][STREAM_AUDIO]->lang;
+
+    const char *ret = NULL;
+
+    // Otherwise, we may be using a filter with multiple inputs.
+    // Iterate over the tracks and find the ones in use.
+    for (int i = 0; i < mpctx->num_tracks; i++) {
+        const struct track *t = mpctx->tracks[i];
+        if (t->type != STREAM_AUDIO || !t->selected)
+            continue;
+
+        // If we have input in multiple audio languages, bail out;
+        // we don't have a meaningful single language.
+        // Partial matches (e.g. en-US vs en-GB) are acceptable here.
+        if (ret && t->lang && !mp_match_lang_single(t->lang, ret))
+            return NULL;
+
+        // We'll return the first non-null tag we see
+        if (!ret)
+            ret = t->lang;
+    }
+
+    return ret;
+}
+
 struct track *select_default_track(struct MPContext *mpctx, int order,
                                    enum stream_type type)
 {
@@ -589,9 +618,7 @@ struct track *select_default_track(struct MPContext *mpctx, int order,
     if (tid == -2)
         return NULL;
     char **langs = process_langs(opts->stream_lang[type]);
-    const char *audio_lang = mpctx->current_track[0][STREAM_AUDIO] ?
-                             mpctx->current_track[0][STREAM_AUDIO]->lang :
-                             NULL;
+    const char *audio_lang = get_audio_lang(mpctx);
     bool audio_matches = match_lang(langs, audio_lang);
     int prefer_forced = type == STREAM_SUB && !opts->subs_with_matching_audio && audio_matches;
     bool select_fallback = type == STREAM_VIDEO || type == STREAM_AUDIO || (type == STREAM_SUB && opts->subs_fallback == 2);