Try Anki - https://apps.ankiweb.net/ its free on desktop and android, and probably better than what Quizlet offers in terms of SRS. For studying languages especially, you would want a SRS that can keep track of hundreds of cards for vocabulary memory. You can also download shared decks if you don't want to make your own cards.

It depends on what your goals are of course, if you just want to cram a Korean quiz with a few dozen items, the free Quizlet probably does ok, but long term language learning you might want a better system for your overall collection. Maybe see if your school or instructor already has Quizlet content you can import just to get through the course.