1. mkvmerge accepts BCP 47/RFC 5646 language tags in command line
arguments that accept languages (which had only accepted ISO 639-2
language codes before).
2. mkvmerge always writes "LanguageIETF" track header elements. If the
given language tag contains a valid ISO 639-2 language code, it
will be written as the legacy "Language" header element; otherwise
"und" (for "undetermined") is written.
Part of the implementation of #2419.
