ForgetMeNot (available in Google Play Store and F-Droid) is a pretty awesome FOSS flash card/quiz app that I'm currently using for my Korean vocab. It has a lot of options and -- importantly for me -- multiple different kind of tests. Self-testing (basic flash card use), multiple choice (they call it "testing with variants"), and spell check. The ability to "invert" cards, so it shows you either the "question" or the "answer" and you provide the other. The ability to hide the "question", so if you want to turn on text-to-speech for phrases/vocab and have to provide the answer by ear.
It's a very neat app, and is a great replacement to paying for Quizlet, in my opinion.
EDIT: My favorite function, which I could not figure out how to do in Quizlet if it's possible at all, is that you can test yourself on multiple sets! I create a new "deck" for each lesson I do in my Korean workbook, and I like to quiz myself on everything I've learned up to now. In Quizlet, I had to go through each lesson individually. ForgetMeNot let's me press and hold to select as many decks as I want, and it shuffles them all together.
I'm with ya there, subs slowly zap away your money and when you stop paying for it, you have nothing to show for it, nothing you can keep! I'm just an old anki person, it's super ugly but gets the job done. I'll send this as a recommendation if someone's looking for a prettier flashcard app thank anki.
I'm all for one-off payments as long as the licence isn't taken away or the service isn't broken retroactively because a subscription version has come out, now that's the worst for me!
Truth! I miss the days of actually owning the things you pay for... This whole license structure big tech companies have gotten us all used to really sucks. Feels kind of like a sword of Damocles, just waiting to drop one day. Everyone's fine with it because "just don't do anything stupid, and they won't take it away from you..." until they do.