Speaking as a total ignorant from a coding perspective. But I guess that wouldn't be the hard part, considering that most of Duolinguo is just boxes and text inputs. How difficult it is to create a database of competent linguists with an efficient training who can progressively enhance your understanding of languages?
+1 for LT. The guy that runs it certainly has an open source ethos. The German one despite being a "Complete" series is frustratingly very incomplete, but that aside it was a useful way into the language. The word order explanations were particularly good. Everything is always free and the project as a whole is expanding with the help of volunteers and donations. It's a good thing to be a part of.