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?
Yeah and their staff have been very dogged about promotion in language communities, including crowdsourcing the content (i.e. getting their users to produce it for free)
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.