one word: anki

Italki is the best one.

It depends - italki for classes, Tandem for tandem exchange. Classic flash card vocabulary and crash course games don't work ťhat well for me, I always stop using these soon.

Anki is the best. But an app is not a substitution for daily effort in the real world. Good luck!

What's up Vlada? If it's for vocabulary, I like Mango Languages. The app breaks down phrases and gives you words and teaches grammar a bit, without it all being very overwhelming. :)