Suppose that there is this question in English posted in the Spanish Stack Overflow. Or a question in Spanish posted in the English Stack Overflow. Is it allowed to edit this question by translating it?
This might seem helpful, but unless you have indications the author actually speaks the site language, you shouldn't do it. As described here:
Please do not translate posts for the OP. They need to be able to respond to feedback, and if they cannot themselves translate the post we cannot be certain that they can understand any feedback provided (by comments, answers, or Help Center content).
It's better to leave a comment (in the original language; Google Translate works fine) letting them know they that should post their question on the Stack Overflow site dedicated to their language.