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?
Well I used Anki for quite some time during my studies and I really didn’t like it.
I think it’s highly personal what suits you best but I really like the idea of a rewarding system or some fun repetition options just like Quizlet has it.