"Which century do we have?"
Translation:Który mamy wiek?
This sentence is extremely weird. The only possible opportunity that you would have to say it would be if you were taking a class and had to do a group project about a given century in time. Then you may ask which century your group has been assigned to.
If we are asking about which century it currently is, then you would ask "Which century are we in?" or "what century is it?"