This is subpage of MCF-specification.
We use 4 octet language codes in various places. The purpose of this is, of course, to allow players automatically select the right language.
The first octet contains flags - well, only one actually: if bit #7 (the highest bit) is set, the language is not original, but a dub/translation. Other 3 characters are ISO 639-2 language code in ASCII. Use "und" (undetermined) as default, if you don't know the language.
List of codes: http://lcweb.loc.gov/standards/iso639-2/englangn.html