The language for https://musicbrainz.org/recording/91b7e04e-529b-4119-bc30-867db56fd400?tport=8000 comes down as "zxx", presumably because one of the 31 works is marked as an "instrumental" performance.

1. もってけ! セーラーふく (partial, instrumental, medley)

However, many of the 31 works have a lyrics language, e.g.,

3. 寝・逃・げでリセット! (medley)

Ideally, the language should be the unique set of all the lyrics languages.  (In the event there is an actual language in the set, I would personally then drop "zxx")