matroskaenc: Don't write a track language tag

"language" is not an offical matroska tag.
Track languages are specified with the MATROSKA_ID_TRACKLANGUAGE ebml.
Writing the tag overrides the ebml specified language during playback with
libav and some other players.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/matroskaenc.c

@@ -1005,7 +1005,9 @@ static int mkv_write_tag(AVFormatContext *s, AVDictionary *m, unsigned int eleme
 
     while ((t = av_dict_get(m, "", t, AV_DICT_IGNORE_SUFFIX))) {
         if (av_strcasecmp(t->key, "title") &&
-            av_strcasecmp(t->key, "encoding_tool")) {
+            av_strcasecmp(t->key, "encoding_tool") &&
+            (elementid != MATROSKA_ID_TAGTARGETS_TRACKUID ||
+             av_strcasecmp(t->key, "language"))) {
             ret = mkv_write_simpletag(s->pb, t);
             if (ret < 0)
                 return ret;