I did 5 years of French throughout junior and high school. I didn't quite reach fluency, but over a decade later without any further instruction I can still read a good amount, absorb new vocabulary quickly (and retain it pretty well -- I just wish I knew about Anki back then), and understand well-enunciated/slower speaking people. (Favorite recent example: https://www.youtube.com/watch?v=WLsxYlTA9Ds He's basically talking about why people find it easy to understand him vs. some random Parisian.) I've forgotten some of the grammar but if I ever get motivated to refresh that I'd go through the book we used in the final year again and make an Anki deck. (https://www.amazon.com/Fois-Pour-Toutes-Hale-Sturges/dp/0801... -- Une Fois Pour Totes. If you can reach the level by whatever means where you can start a feedback loop of teaching yourself more within the target language, I think it helps a lot. Even using a French-to-French dictionary, rather than English-to-French.)
In contrast to French I've been lazily teaching myself Japanese over the past few years, but I haven't been diligent about it and thus my conversational ability is still basically 0, my vocab is limited (though recently expanding quicker as I'm able to pick out more and more words/phrases from media), and my kanji count is only ~500 out of 2000. With more diligence I think I have enough books and methods to synthesize them that I could achieve similar proficiency as my current level of French even without a classroom environment to test it in, but I'm also fighting prioritization. If it were higher priority I think I'd try and enroll in a community college class or something just to get a boost on speaking with immediate teacher feedback.
TLDR: As far as free tools go, Anki is great for any language, especially when you build the decks yourself. (I like the existing "french core 3k" deck as it includes audio for its sentences, but unless you already have a basic understanding of French grammar, I think it might be too much to start with.)
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