I do not see how this could possibly mean "I am on the toilet!" The verb is "van", not "vagyok".
I have only heard and read signs for WC in Hungary, and mosdo is for washing. Sophisticated, you have my attention, thanks again Shamarth
If there's a toilet and nothing else, then it's not a mosdó. But as soon as there's a washbasin too, mosdó is an appropriate name. It's also more... sophisticated to call a room like that "mosdó" instead of "vécé". I always say "Kimegyek a mosdóba" instead of "Elmegyek a vécére."
Why do you use "kimegyek" in combination with "mosdóba" and "elmegyek" with "vécére"? Is that just how you say it or is there an actual reason for it?
Those phrases felt the most natural to me at the moment, but "elmegyek a mosdóba" and "kimegyek a vécére" are equally correct.
maybe "lavatory" is a better translation for "mosdo" into English than "toilet"?
It is probably a more accurate description of "toilet plus wash basin", but mostly in English I understand "lavatory" to just be a euphemism for toilet. I understand that for many people, especially those from North America, hearing "toilet" evokes the image of an actual receptacle for human waste, but its perfectly polite in Australian ( and I dare say NZ and British) English at least, to say, "Excuse me where is the toilet" or "I just need to go to the toilet" i.e. the entire room, including basins, hand towels whatever is "the toilet". I don't think its helpful to be too restrictive here with the acceptable translations.
Calling anything a WC is unheard of in much of North America. Just like we dont call elevators "lifts".
Guys, that's good, but for example I am not a native speaker and I never heard the word "lavatory" until now. So for me "toilet" or "bathroom" were way much better, bring it back please!