It's a for profit public company. I've been playing around with the idea of possibly creating a FOSS app that would run its servers via donations and (same as duolingo in the beginning at least) community created courses. Would need support from proper linguists though.

There are other apps like that but they lack the sleek UI and gamifying aspect or simply have only one language.

I pay for duolingo so I hate that they are tracking me but it's the only actually fun casual language learning app out there and honestly I don't mind paying for such a good app, but their monetization schemes are getting more aggressive: e.g. time limit games which are nearly impossible without paying for "extra time potions" which you don't get even if you have the paid version.

Edit: just to add to the time-limit games: you can just not participate in them, but fuck me for paying to the app and still having that bullshit in my UI.