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?
In a FOSS app, that would be completely fine.
Make a gamified app for language learning, accompanied by a gamified app for creating the database. Maybe similar to StreetComplete. This could also be used to check/resolve translation errors reported by users.