I am amused by duolingo's fondness for the word WEIRD, with its echoes of the supernatural!. Latrine, lavatory and toilet are all euphemisms, all with the basic meaning of place to wash and each replacing the former in turn due to the hypersensitivity of the English middle classes. I have seen the word 'lavatory' used in an official US Government document for hand-washing facilities in meat processing establishments. Bathroom, however is a room where one can have a bath and while often used as another euphemism for 'zachod' I believe that it would be best avoided in that context in Czech lessons. I would suggest WC!
Might as well throw my two cents in here, too... in the interest of expanding our learners' familiarity with US usage related to "where to go." (Caveat: Times, however, are a-changing on this front).
In the US, in a public place like a restaurant or bar, I think "restroom" would be the most inquired-after term, followed probably by "bathroom" and "toilet" in that order. I wouldn't expect most Americans to inquire after a "lavatory" or -- maybe except for those who have traveled outside the US -- a "WC."
I am familiar with "lavatory" as a word used in some public institutions (e.g., schools), in which use it has referred to a "gendered" room equipped with sinks, toilet-containing stalls and, where appropriate, urinals.
In a private space, like someone's home, I think that most people, when looking for somewhere "to go," would ask for either the "bathroom" (which generally also contains a bathtub or shower, along with a toilet and sink) or the "powder-room (which usually has only a sink and toilet.)
By comparison, the Czech koupelna and záchod now seem way more clear and direct. :-)