alt Hacker NewsI've had excellent results for French with a series of books called Grammaire Progressive du Français. Self-contained lessons of grammar. Vocabulary is easier because I already spoke a language with significant cross-pollination with French vocabulary.
It's not an "app" and doesn't have a "streak" or an "HP bar", so...
And if you really like to suffer, you can do it like the locals and try working through a copy of le "Bescherelle"..