I'm sure this is a wonderful project with talented people behind it, and what I'm going to say isn't a criticism of this project in particular.
But. I'm always a little disappointed when I see a project that's Libre[something proprietary]. It's always a wonky copy, where the selling point is that it's a free version of something, rather than a better version of something. The only people who are going to use it are those who care more about the fact that it's free and Libre than they do about a good learning experience [0]. Everyone else will just use Duolingo. And that's fine if the goal is for it to be a programming exercise, but it's a limiting one.
Instead of making a knockoff of Duolingo, which clearly been eaten by the pressure to drive engagement and MAU, why not use time and energy to explore different or more radical ways of online pedagogy free from commercial pressures? It's harder than copying something, but the results could be much more worthwhile. [1]
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[0] This is why Mastodon will never go mainstream, because it's built by and for people who care more about decentralisation than they do about creating a first-class microblogging experience. The friction points that deter the mainstream are acceptable for the true believers because for them the benefits are worth it.
[1] This is also my problem with Linux desktop environments. The desktop war was won by Microsoft 30 years ago and the desktop died as the primary computing paradigm in 2007. Yet Linux desktops are still fighting the last battle - so much time and effort is poured into them, yet they still don't work right (Wayland is how old now?) and are basically just wonkier versions of macOS or Windows.
Surely that time and effort could be spent on investigating new ways to interact with computers - why is the desktop metaphor still the best we've got, nearly 60 years after it was first invented?