Ask HN: Is building a calm, non-gamified learning app a mistake?

Not doing subscriptions for an app that has ongoing server costs is going to bite you, you may want to reconsider that.

Your biggest issue is going to be that language learning for adults is largely an unsolved problem. I know people with 1000+ day streaks on Duolingo who are nonetheless not fluent, and from everything I’ve read, it seems clear that spaced-repetition techniques are not sufficient (and possibly not necessary) to achieve fluency. Most people say you need immersion, which is difficult for an app to provide (research other people who have tried, you probably wouldn’t be the first and can save a lot of time, effort and heartbreak by learning from other people’s failures).

It is a solved problem, Assimil or Michel Thomas (in person) have been making people conversational within a week of dedicated study. Plenty of language learners have reached C2 in 6+ languages including me. But it's not fun and certainly not appable (and these apps never offer anything beyond flashcards)"appable".