Alternatively, in the languages I speak:

Welche Sprachen sprechen Sie? (Deutsch/German)

¿Qué idiomas habla usted? (Español/Spanish)

Quelle langue parlez-vous? (Français/French)

EDIT: These sentences are now up to date.

        • Oh damn. It didn’t even occur to me that we were talking plural here lol

          Obviously you’re right.

          edit: I honestly hate the fact that English doesn’t have a non-vernacular way to distinguish between singular and plural in the 2nd person. Makes it so much harder to get my head around this sort of situation. “What languages do yous speak?” Would make it so much easier!

            • Yeah, sort of. I also use “yous” frequently as part of my dialect regularly. But it’s certainly an informal usage that I would not normally use in written communication.

              I actually suspect, though I haven’t investigated it enough to be confident, that there may be something else going on. That there’s possibly a difference—in my dialect, at least—between 2nd person plural “multiple specific people” and “a general large audience”. And that “yous” might only be appropriate in the former.

          • Yeah, it is the hardest thing when learning a new language. When you learn a new concept that your language doesn’t use. For example, in Latin, German and Japanese, the grammatical case is very important but totally irrelevant in French and English. So I try when I speak French or English to think about the case. That way it comes more naturally to me when speaking German or Japanese.

            • Yeah, the catch here is that it’s a feature that my native language does at least sort of have, just applied in a way that makes it not clear. When it’s a feature I’m completely unfamiliar with, I’m more likely to be on guard for it, if I’ve learnt it. But here I didn’t even think about it, because it was an element I am familiar with, so I never second-guessed my intuition, even though that intuition was wrong.