Certificates of Proficiency in a Language?
Will duolingo provide official certificates of proficiency in a language in future?
I don't know if that's something they can legally do. Depending on your country of origin, there are probably official tests you can take for that.
If you want to, you can take a language test here: http://www.telc.net/en/examination-centres/exam-registration/register-online/
Like eldest said, an official certificate would require an official test...perhaps duolingo could get licensed to administer them. A little reward after completing an entire language would be nice, though. :)