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?
Conversely, I’m at a similar level in Dutch and I’d attribute a lot of my grammatical knowledge to Duolingo, especially modal verbs. However I have scoured dutchgrammar.com many a time because I learn well from reading textbooks. It hasn’t helped me at all with output though; I was quite bad at that until I moved to NL.