Your site is bound to attract "amateurs" who are just learning languages for disparate reasons. Nothing wrong with that, but a better analog to StackOverflow would be a forum embedded in a website for professional translators and the people who hire them. IIRC http://proz.com does have a simple forum for that but it's not very good.
I'd volunteer to answer questions on Portuguese, but there are none right now. I might check a few more times later. But that's just because I saw it here on HN and would be glad to help you getting it off the ground. Otherwise I wouldn't bother -- unless it was part of a professional translation site and participation had a positive effect on my chance of getting translation jobs.