I came across a question in another language that I happen to speak. Should I edit it and add a translation?
If it's a good question and you have more than 2000 rep points*, I don't see why not - if you manage to edit it in time before it gets downvoted and closed, which will usually happen quite quickly.
Of course, what Servy and Robert Harvey say applies - it's doubtful whether the OP will be able to follow the conversation in English, and trying to save a question this way is "heroic": the chance of a positive outcome can be very small. It's probably not something you want to do on a regular basis.
* as @dcaswell notes in the comments, suggesting a translated edit is a hopeless endeavour. You should be able to do direct edits if you want to translate stuff.
No, for a few reasons.
First, we can't rely on you (or other bilingual users) to always be there to translate questions for us, so such questions will get unequal treatment. Google Translate is available for those who want to make the attempt.
Fundamentally, SO is an English site for English writers. Users who communicate in a foreign language are going to find it difficult to respond meaningfully to the community.