Why don't the owners of these various Q&A sites just go multi-language? It's pretty simple, if you've got a community behind it.
- a multi-lingual user would dillute his effort , and most likely not bothering to answer the same question in both languages;
- english is the lingua franca of programming anyway - see 'The Ugly American Programmer;
Eu posso ajuda com portuguese (brasil).
Because I'm useful only for questions asked in English, and for giving answers in English. If we all have a common language, we can all participate in our areas of expertise, and we can come to reasonably agreed answers. If this were a multilingual site, then we'd reduce it to a batch of overlapping sub-communities, and we would have a danger of having significantly different answers in (say) German and Japanese.
By having one official language, we have one community, with sub-communities based on subject area rather than language, and we can consolidate duplicates to some extent. The benefits are real. If we have only one language, it obviously should be English. (This isn't true for all conceivable subjects, but it is for software.)
I'd have absolutely no objections to having easily available translations of questions and answers, since there's a lot of people using this site that have problems with English (and I really, really hope that all of those people are not primary English speakers). Really, I have nothing against Swedish speakers, I just can't communicate with them except in English, and so I can't cooperate with them except in English. (Yes, I'm a US citizen.)