Not true! I was already fluent in English way before living in US. But living here def improved my vocabulary and pronunciation.

I studied Japanese in Tokyo, and yes I guess it’s easier to learn and remember because I did not have a choice but to use them and practice in my everyday life, since not so many Japanese speak English or any other language. Btw, our Japanese teachers did not speak one English word while teaching, but somehow we understood and learned the language.