Eluveitie has a number of songs in the extinct Gaulish language. It would be nice to have it reflected in the database.
However, what makes things complicated, is that they use "a reconstructed form" of Gaulish, rather than any one of the (fragmentarily) attested forms codified in ISO-639-3:
xtg - Transalpine Gaulish
xga - Galatian (can be ignored here due to geographical placement in present-day Turkey)
xcg - Cisalpine Gaulish
xlp - Lepontic

Looking at Eluveitie's lyrics, it indeed looks like they have used sounds (xs, nd, word-final m) which have become variously assimilated in the individual variants (according to Wikipedia anyway: https://en.wikipedia.org/wiki/Gaulish_language). Since ISO-639-3 doesn't have a code for Gaulish macro- (or rather, proto-) language, maybe an ad-hoc code (i.e. in the qaa-qtz range) can be used in the database, e.g. qga or qgl?

List of songs in Gaulish:
https://beta.musicbrainz.org/tag/gaulish/work