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?
I'm not convinced that model of learning is really effective anyway. At best, it's a fun-ish time killer where you learn a bit about language.