"Why do you not speak Polish?"
Translation:Dlaczego nie mówicie po polsku?
"Po co" means literally "What for", so that would definitely sound weird.
I remember such examples from the Russian course, so I will adapt them and translate into Polish:
"Dlaczego rozbiłeś szklankę? Ponieważ była za gorąca." = "Why did you break the glass? Because it was too hot."
"Dlaczego (po co) rozbiłeś szklankę? Żeby pokazać ci, że jestem wściekły." = "Why (what for) did you break the glass? To show you that I'm angry."
Ah, now it is marked as correct. Previously, I swapped "nie" and "mowisz" ... but shouldn't it be still correct?
It would make some sense, but not really here. "Mówisz nie po polsku" is "You're talking in some weird language that is definitely not Polish". Most probably "You're talking theoretically in Polish, but using some weird words and/or structures." ;)