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 think a combination of fast advancements in multilingual open source TTS models and maybe LLMs? could help. The problem is LLMs habitually lie. It also may be unneeded. Duolingo seems relitively simple iirc (2019 was the last time I used it) a combination of simple phrases and good gamification of topics. Add some anxiety and there you go, Duolingo lite or sumthin