I learned "Kung Fu" mandarin when I trained with fresh off the boat professional martial artists who didn't speak a lick of English. The only way to communicate was for me to start learning mandarin.

That said, I believe there's no better way to learn a language than to live in the country and not speak English.

I was a diplomat brat and at various points in my life I could speak Bahasa Malay, Italian, and Hokien (does anyone even know what that is). However they all fell into disuse when I moved. But the reason I became fluent was because I was immersed in the people, all the kids, TV, everything.

I know many American teachers in Thailand who live in American neighborhoods and eat where foreigners eat. As a result they never learn any Thai.