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 ✳️ 

True of Mandarin as well. Confirmed by my irl tutor. DL used to have community notes on each question where native speakers would chime in and help correct some of the framing. Looks like they might’ve got rid of this feature recently though.