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.
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.
Men commit almost all rape and murder, but no one seems to think this is a problem we need to do anything about. If any other group committed 90% of serious crimes – let's say immigrants – people would be calling for them to be rounded up and exiled. But when it's men, that's just the way things are, nothing to be done about it.
But we know exactly what to do about it when we're talking about other species. We castrate male cattle because bulls are dangerous and steer aren't. Violent criminals typically have elevated testosterone levels. It doesn't take a genius to realize that putting men on testosterone-blockers is going to make them less dangerous.
What will be the other effects?
Men will be be weaker—Meh, physical strength is less useful now than it was in the past.
They'll have less hair on their bodies and more on their heads—Awesome.
They'll won't be able to get hard—That's what viagra is for.
They'll lose fertility—They can go off blockers while they're trying to have a child. Or you could just have a small number of uncastrated sperm donors.
I'm an anarchist, so I don't want to force this on anyone. But if I believed in prisons or police, I would also believe in mandatory castration.
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.