It depends on the language tbh. Several languages on duolingo are decent ways to dip your toes in the water for language learning, I’ve heard good things about the Spanish, German, and Mandarin courses. But it’s really dependent on the language, I tried Duolingo’s Irish course and it was so bad that I’m still trying to unlearn a good amount of misinformation, really soured language learning for me even though I eventually got a good course elsewhere.