"Excuse me, who are you?"
Translation:Извините, кто вы?
«Извини́, кто ты?»
You definitely should report this as an error next time you get this sentence.
Are you sure about that szeraja_zhaba
Can a stranger use the informal ты?
Yes, this works in some cases:
- if you are in an informal group of people where everyone is considered a peer (e.g. a group of students in the university; this might be acceptable at some parties, in some websites, at some workplaces — this is very individual, though!),
- if you are talking to a child,
- if you wants to deliberately insult someone and show your disrespect.
I do not have native fluency in Russian, but to me the former is asking for a name whereas the latter is seeking explanation of relationship or position. (In English as well, though depending upon tone, could definitely be taken to be stern or as a sign of mistrust.)