"Do you want to go to the bathroom?"
Translation:Chceš jít na záchod?
In most English speaking countries we say toilet when we mean toilet. The bathroom is a place where we wash and bathe, although it may also contain a toilet. This translation is only correct in American English. Perhaps they find the word toilet too vulgar.
Therefore koupelny should be accepted here, especially since it is possible to go to the bathroom to take a shower or brush one's teeth, and many bathroom do not contain a toilet.