"Are those three girls Czech or Slovak?"
Translation:Jsou tamhlety tři holky Češky nebo Slovenky?
Would it be completely wrong to say "Jsou tamhlety tři holky ceské nebo slovenské"?
It would be somewhat unusual, but perhaps acceptable, I will have to consult the others or check the corpus. You would certainly be understood but it is not how people normally speak. Maybe in some situations where there are several international groups around and you are asking which one is that one.