I'm working on spaced-repetition language flashcards app, that shows you variety of card types (sentences, reverse, audio, definition...) and allows you to add vocabulary from content - youtube, ebooks, website reader.
What’s baseless about it? The yearly subscription price is like 20 minutes of work for programmers and many users are spending tens of hours on the app.
ios: https://apps.apple.com/us/app/vocabulary-flashcards-vocabuo/... android: https://play.google.com/store/apps/details?id=world.petr.vok...