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.

I quite like busuu for Spanish so far. It is a lot better than Duolingo trash as it actually explains things rather than just throwing stuff at you that is often incorrect anyway.

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