Has everyone said we should answer the question with the language that was used to make the question.
The problem is, you will get questions that are duplicated but in diffrent languages, take this one for example. This question may be a duplication of the english question Do foreign acronyms take a definite article, and of which gender?
So the solution would be answering in both languages but then we got another problem, the user knows the answer in English but he doesn't knows how to write in Portuguese, this is unlikely to happen since most of the users knows how to write portuguese, but there's the other way, the user knows the answer in Portuguese but he doesn't knows how to write in English.
This takes us again to the first problem, should we really consider the questions duplicate even when they're written in diffrent languages?
To finish, we should first answer the question with the language that was used to make the question, then we could answer in the other language but only if we know to write. If we don't know how to write, the community could edit edit the answer an add the answer in the other language.
This would be a possibility but imagine long anwsers, I guess we would write in one language and then if someone else asks that question in the other language we would answer and not consider them as duplicated.