Translation:Who is in the toilet?
This phrasing is kinda unfortunate, unless the person is actually insidet he toilet bowl.
Could indeed be. But without context, chances still are that they are at the toilet.
In British English you would commonly say "Who is in the toilet?" since toilet is another way of saying bathroom/restroom. In American English, the toilet is the toilet itself so you'd visualise someone standing in the toilet bowl.
No, definitely on the toilet, unless you aim to stick a person inside of a toilet.
You are right, in American English. However, "toilet" also means "bathroom."
Maybe in some regions. You would never call a bathroom a toilet though in my experience as a native speaker, and you would definitely not ask who is IN the toilet.
No, in. The toilet is a room (in this context) and we are never at a room. The This sentence is entirely correct, though not idiomatic in American English (for a change!)
I certainly hope that we won't start accepting 老子 as a correct answer for "father" one day. Or even worse, 阿嗎 or 爹爹. And you know they are all correct as far as Chinese is concerned, if we want to be this inclusive...
Just kidding! I know what you mean.
So the actual toilet bowl is 馬/(马) 桶 (mǎtǒng). This will be VERY useful I'm sure.. /s
Actually, it is more colloquially appropriate to use 上 instead of 里 because 厕所 does refer to the actual toilet. Usually, if you want to refer to the room itself, you say 洗手式(ch.) Or 洗手间(tw.)