A few days ago I saw a “language” in the list I had never even heard of: Lolspeak
Obviously, Lolspeak is not a language that “builds bridges”, since it’s just English with a lot a spelling errors.
In general, though, I would say keep all genuine languages. And I don’t necessarily mean only “official” languages, but languages that could be someone’s mothertongue. I wouldn’t like to exclude anyone, just because their language is “not important enough”.
I do get your problem, though. It can be hard to check the sincerity of volunteers of less known languages. But if all CM’s who didn’t speak Dutch hadn’t allowed me to be in their team, I would have had a lot less subs than I do now.
You might consider keeping the “suspected” languages away from your team or maybe find some way to check them. But totally removing them as an option doesn’t seem right to me.
Btw, I’m pretty sure Frisian users would love Frisian subs. 