Recently, I stumbled upon this post, which was posted in Portuguese. When I asked the OP in a comment to restate the question in English, other posters told me that according to this post on Meta, questions in other languages than English should be allowed.
While reading that Meta thread, I couldn't find an actual conclusion to the discussion, but I did see some good arguments to both sides of the story. After reading the thread, there's one major issue that does not seems to be discussed and that bothers me to an extend where I think it deserves its own thread. So, let's just assume that we all totally agree with allowing non-English posts.
One of the main goals of this site is to help people in their pursuit of learning mathematics, in helping them understand those parts of the discipline they find hard. If someone asks a question in a language different from English, this will mostly be due to one of two reasons. Either OP doesn't know any English, or he doesn't trust his understanding of the language to the point where he's willing to actually write in English.
Now suppose it's actually a good enough question to get translated. It then is not unlikely that it receives some decent answer, written in English. At this point some of the main points for this site are satisfied: a good question has been posted, recognised as such, it has been answered and in the process it has entered the extensive library of decent questions on MSE that might be helpful to others as well. However, the answer is written in English and the OP will either not be able to understand it at all or have an incredibly hard time to understand it, without getting any of the nuances in the answer at all.
When a question arrives at this stage, the OP should be able to benefit from having asked a question and having received an answer, which, so far, he cannot. I believe that at this point the answer should be translated back to the original language, so that OP can have all the benefits. Moreover, by translating the answer back, we might obtain a useful member to our community with great questions and/or answers, while otherwise he might be disappointed by the lack of understandable response and leave forever.
At the current time, the whole process of MSE, even if we allow for posts in other languages than English, doesn't seem to provide for situations like this, even though it could be a valuable extension. What do you think? Should this process be accommodated in a multilingual environment? If so, how could we achieve this? And if not, do you think we should allow non-English posts at all?