I'm originally from Midwest, but live in Tokyo for the last 6 years.
I'd say Japan is not too different. Yes, by being mainly mononational country, contrary to the US, you won't be treated as a local even if you're fluent in Japanese. But to tell you the truth, there are not too many places in the world were you will be treated like local, even if you live for a more or less considerable amount of time.
But the thing is, it's not that important to be treated like local to make friends. On the contrary, I'd say. A lot of people are genuinly happy to meet foreigners, especially those who try to learn their language. So it's really quite easy to become friends with Japanese people, despite any cultural differencies you might have.