Great to see a FOSS app for language learning! Kudos.
However, I think apps that focus on one particular language and how to learn that language are better than a one-size-fits-all approach like Duolingo. The structure and grammar of languages like Russian, Japanese, Chinese, and French (I've learned all 4) are all significantly different from each other. Or at least different approaches for language groups (French and Spanish, which I also speak, are similar enough to warrant the same approach).
However, I think apps that focus on one particular language and how to learn that language are better than a one-size-fits-all approach like Duolingo. The structure and grammar of languages like Russian, Japanese, Chinese, and French (I've learned all 4) are all significantly different from each other. Or at least different approaches for language groups (French and Spanish, which I also speak, are similar enough to warrant the same approach).