The more popular something is, the less effective it is to criticize it harshly. For example, I think eating meat regularly is, by the amount of suffering it causes, worse than murdering one human. But if I went around calling everyone who ate meat "murderers" and refused to befriend or do business with them, it would just make people think I was crazy and not want to listen to me, because eating meat is seen as normal. On the other hand, when something is seen as abnormal, like being openly racist, shunning people who do it makes others less likely to do it too.
The best app is whatever you use to consume media in the target language.
The only way to acquire a language is to expose yourself to it in a natural context. You can't acquire a language just by studying with flashcards or grammar exercises or whatever – any app that offers enhanced versions of those will at best be a minor supplement to actually using the language.