A general point: not all foreign language users need to pass for locals --- or, not all foreign language use-cases require that. An extreme example (not one t

A general point: not all foreign language users need to pass for locals --- or, not all foreign language use-cases require that. An extreme example (not one that Duolingo caters for) is opera singers (and singing in general): they need flawless diction, and thorough understanding of the text, syllables, and how they map to the meanings that they are singing. But they don't need to speak, read, write, or understand the language for day-to-day communication --- and there are anecdotes about being cornered in (eg) Russian, and having to 'fess up that they don't actually know how to chat in the language.