It does seem a weird choice. Having the user's default be "Undetermined" makes sense, but having that selectable as a choice when entering kind of doesn't. And an "All" dropdown would make more sense for searching rather than having "Undetermined" along with all the other choices.
Okay, I found an answer to this. Since most content on Lemmy doesn't get tagged with a language at all, selecting anything other than "undetermined" can cause issues.