I will start by referring to a question on MSE (Math Stack Exchange) (this) and an answer on Physics Stack Exchange Meta (this). Let me cite the questioner on MSE:
Imagine I want to ask an extremely precise question with a lot shades in one of all possible subjects on Stack Exchange but I am not fluent in English. Can I find someone here to help me to translate my question?
My question is much the same - but instead intended for Physics Stack Exchange. Also, let me cite the answerer on the Physics Stack Exchange Meta:
I do agree that multiple languages on one site cause confusion. But we should not "ban" non-english speakers, ... try to translate things to english as good as possible. As soon as the correctness of the translation is verified, the original text should be removed - it will still be available by clicking on the edited by link. Maybe one can edit in a original version in russian ...
But this is not, at least not according to me, an answer that clearly tells Physics Stack Exchange how to behave should this situation occur. Instead, someone tells you how you should behave.
Please forgive me if I have missed some already written rule on Physics Stack Exchange about what I'm asking for here.