For the past 60d I've been using Anki (a flash card program) and it's FSRS setting to learn my French deck (5000 most common French words) and I'm absolutely zooming. I can already follow a fair chunk of conversational French.
I've also been using the same system to learn Chess more deeply (endgames, tactics, openings) through Chessable and a few other websites that offer FSRS. It's levelled up my chess game a lot
Basically - the thing that hooked me was the data. Being able to see how many cards I've reviewed, how many cards are at 90/80% retention, the stability of every piece of that knowledge, the decay rate, etc... It's really cool.