I disagree because of how AI is progressing and because there's tons of neglected language markets they can pick up. Obviously your approach can work too, perhaps better. But 95% of language learning tools don't support Thai (my target language) for example so I am an eager user for that reason alone. I think they'll be able to make a generalized curriculum and have the AI use it in all languages.
Would you rather have a tool that teaches you accurate conversational Spanish ?
Or something that tries to teach 60 languages but does so poorly ?
My tool supports Thai, if you'd like to try it - https://nuenki.app . I added it at the request of a user, who seems to be happy with it.
It's a browser extension that finds English sentences in webpages, and translates the ones at your difficulty level into the language you're learning.
Most of the generalized curriculum stuff out there is crap because languages differ from each other in substantial ways. LLMs in principle should help here as they can use their knowledge of the structure of the language to modify, but we're just not there with context windows and thinking capabilities. They will need at least a per-language (ideally per language pair) system prompt that contains a rough outline of the curriculum.