I haven't tried Duolingo in the last couple of years, but I thought it wasn't the best to begin with a language.
It's pretty good to learn basic vocabulary, but I felt you would need something before, just to get some grounding in the language. Like basic grammar, and a very high level overview of how the language "works". You can infer some of it from the example, but that's a lot more difficult than already having this context in my opinion.
- Duolingo(don't do it for more than a few months)
For beginners
- https://www.languagetransfer.org/free-courses-1#french
For all others, some sort of immersion approach is the only way if you don't live in the country. Figure out the general approach below and adapt for your target language.
- AJATT http://www.alljapaneseallthetime.com/blog/all-japanese-all-t...
- TMW https://learnjapanese.moe/
- Refold https://refold.la/