Language attribute in MusicBrainz currently relies on ISO 639-3 (mandatorily) and supports ISO 639-1 and ISO 639-2 too. Actually, ISO 639 is an industrial standard (that partly explains difficulties for its funding). As such, it might be not the most appropriate to document lyrics.

On the contrary, Linguasphere Observatory is a linguists-driven non-profit organization which attempts to cover all recorded languages and their varieties. It is the policy of the Observatoire that its on-going independent work on language coding should be complementary to and supportive of the ISO 639 international standards. Source: Wikipedia. There also is a Wikidata property for Linguasphere code.

Having support for Linguasphere code additionally to existing support for ISO codes would allow to handle Gallo, Jacaltec, and other requested languages.