Ankidroid is what I use for studying languages.

Dan, if you are still playing with Obsidian then you may be able to just use that. I see a 2021 post for a plugin for flash cards. I'll paste the link here. In-app in the plugins the app lets you search.

https://forum.obsidian.md/t/plugin-for-flashcards-note-level-spaced-repetition-all-inside-obsidian/16498

I cant recommend an app. There are some on F-Droid though.

Anki. spaced repitition