From my experience, spaced repetition is only really useful as a trigger to recall prior meaningful experiences. The really tricky part of using spaced repetition well is writing the cards: you need to work with new material until it feels obvious, and only then make cards that will recall that experience of it being obvious.

For language learning, there’s one other wrinkle: you need to be working with material written in the target language, rather than written about the target language. Most of my language cards are Cloze deletions from novels I’ve read or museum captions, for instance.