https://musicbrainz.org/doc/Style/Release#Language_and_script

The language attribute should be used for the language used for the release title and track titles.

There is a difference between "written" and "spoken" language, but the guidelines ignore it.  For example, "Mandarin Chinese" is a spoken language, not a writing system.  I would expect the language to just be "Chinese" (with either simplified or traditional indicated in the Script field): https://en.wikipedia.org/wiki/Mandarin_Chinese

I think the guidelines should say that the Language field should be restricted to written languages, not spoken languages.