Here is an example of a release, whose back cover prints the tracklist in Latin, but whose medium labels print it in Japanese:

https://musicbrainz.org/release/3a08cac9-25a4-4add-b35d-7af3191753cf/cover-art

Which of these should be used when entering the tracklist?

From what I've heard asking around informally, there's no particular style guide (or even consensus) about which way to go with a discrepancy like this, and it seems like it'd be nice to settle...

I'm tempted to say it could come down to something like "go with whichever is in the language's own script", though that would only settle cases like this one where the discrepancy is between two scripts, and not ones with another type of discrepancy (like long vs short names for songs with sub/alternate-titles).

If we really are settling it purely between "medium or back cover", I'm tempted to go with the back cover, since that's the one that most people would be more familiar with: the medium is generally either going to be inside its case or spinning around too fast to read, so most people consulting a tracklist would generally be looking at the back of the packaging to do so.