Duolingo and other apps are fine to build passive vocabulary (the words you can understand) up to some level, but the real issue for me is the lack of a real "speaking" component.
Frequent speaking practice is key to build your active vocabuary (the words you can use). It's also key to build your confidence in using a language.
Shameless plug: that's why we are making an app that really makes you speak: https://en.globers.co/
I just bought a 2-month expiry put option at a strike of $10k on your car.
Two month later, I check the resell value for your car.
If it's more than $10k, let's say 13, I buy yours at 10 and resell it at 13K. I won 3-1=$2k
If it's less that $10k, I just pass, I lost $1k.