demux: mkv: check the language string is a valid ISO639 one

Even the IETF base languages is using ISO639.

modules/demux/mkv/matroska_segment_parse.cpp

@@ -39,6 +39,7 @@ extern "C" {
 
 #include "vlc_colors.h"
 
+#include <vlc_iso_lang.h>
 #include <vlc_codecs.h>
 #include <stdexcept>
 #include <limits>
@@ -1103,6 +1104,9 @@ void matroska_segment_c::ParseTrackEntry( const KaxTrackEntry *m )
 
     if ( bSupported )
     {
+        auto *iso_lang = vlc_find_iso639(metadata_payload.lang.c_str(), false);
+        if (iso_lang != nullptr)
+            msg_Dbg( &sys.demuxer, "Unknown language %s!", metadata_payload.lang.c_str() );
         p_track->fmt.psz_language = strdup(metadata_payload.lang.c_str());
 
 #ifdef HAVE_ZLIB