[steven]

I think you'd learn a lot with the experience. I also can say that while pretty much every Japanese person studies English in school, most can't speak it. They can read and write very well though (the people who take it seriously anyways). Yokohama is a very big city (I guess #2 in Japan under Tokyo... but they're like connected as far as I can tell). So I'd guess that you'd run into quite a few people who can speak English fairly well, as there are a lot of foreigners living in big cities (thus giving Japanese more exposure to natural English I suppose).

However, if this can make or break you getting a PhD, I'd consider all your options before making such a big move.

Out of curiosity, why Japan?