Duolingo is decent but gets increasingly annoying the further you advance. With fancy sentences, there are many correct translations and you have to guess which one it wants.

To test whether it was gaslighting me on Spanish, I skipped to German lvl100 (my first language) and found a good handful of false negatives.

Real World Ethereum 🌎 
building /daimo ✳️ 

I really like the language transfer method, a YouTube series on using the language you know to the one you're trying to learn.  Helped me massively with Spanish while living in PR!