* include/common.h, src/misc/modules_plugin.h: export the 3 new functions.
  * plugins/dvd/dvd_es.c, plugins/dvd/dvd_summary.c, plugins/dvdread/input_dvdread.c: adapted DecodeLanguage calls to the new behaviour.
  * plugins/mpeg_system/mpeg_ts.c: ISO-639-2 descriptors handling.