The problem is duolingo is particularly horrible and is intended to get people addicted, not educate.
Theoretical educational frameworks don't replace the day-to-day struggle of trying to get shit done. (Doing this is, of course, extremely uncomfortable, and people will avoid it at all costs.)
I speak three languages fluently (two prior to moving to said country; English, for example, is not my native tongue), so that's a weird assumption to make. With that said, I still think it's the most efficient way to learn a language, especially given how almost everyone's a nomad (especially in tech) these days.
The point was what's the best way to learn a language other than by having an entire country surrounding you dedicated to that language? Many / most people can't pick up their life for a year to learn a language. People have work, people have families, people have local commitments.
I guess my original comment could’ve been more specific, but I figured the context was implied.