You need to get more specific about your market. General language learning is too broad of a category in order to make informed choices.

Who are your ideal customers -- and how can you serve them best?

"Calm" is not a niche, its a style.

A niche is your position relative to the people you're serving.

Learning apps serve huge varieties of markets, with very different needs. For example, math apps designed to supplement math education in an elementary school environment, versus a language learning app designed for someone traveling to a X country in 3 months...

Your questions should be anchored to very specific audiences that you can actually reach.