Huh. I've had dreams "in Russian", Greek, Latin, Japanese, and Arabic. I know none of these languages beyond a little counting or a few words, but they seemed very real at the time!

I still think that being able to crack an improvised joke based on the situation one is in is a stronger sign of fluency. But it rests on similar assumptions: having internalized the basic structures of the language to the point where you can spend your cognitive resources on execution.