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?
As jet said the difficult part is the content, not really the “technology” (which is mostly just flashcards tech + quick “fill the void” exercises, and we already have software like Anki). There’s probably the gamification aspect too.
There are lots of free language learning materials on the internet. For Japanese for example, there’s Tae Kim’s course: https://guidetojapanese.org/learn/ (his grammar guide has a CC BY-SA 3.0 license, his “complete” guide has no license yet as he hasn’t finished it yet). It would be great if course authors could allow open-source devs to build a unifying app on top of their courses (it does seem that Tae Kim is okay with it, since he allowed an Android dev to make an (now discontinued) app version of the course: https://play.google.com/store/apps/details?id=com.alexisblaze.japanese_grammar).