I'm in the process of adding several hundred Indigenous Australian releases to the musicbrainz, many of which are titled or sung in languages not available in the drop-downs. Below is a large list of languages, although I expect we would only use 10 to 20 of these.

https://en.wikipedia.org/wiki/List_of_Australian_Aboriginal_languages

Alternatively, rather than just add extra languages every time someone creates an issue, maybe musicbrainz could adopt an existing comprehensive standard. In the past I have used the Australian Standard Classification of Languages, which is available for download here below:

http://www.abs.gov.au/AUSSTATS/abs@.nsf/DetailsPage/1267.02016?OpenDocument