Since it is held that all living things have "Buddha spirit" I have found that temples tend to be quite welcoming places regardless of whether one is a foreigner...or even a Buddhist. It strikes me that the upshot of all living things having Buddha spirit would be a viewpoint of there being two kinds of people in the world: 1)Buddhists and 2)Buddhists who don't yet realize they're Buddhists. So why make a distinction between "Japanese" and "non-Japanese"? At any rate, I have always been made to feel quite welcome at temples here and can't imagine that they would discriminate.
As to how difficult it would be to study philosophies in Japanese...wouldn't that sort of depend mostly on your Japanese?
You may already know this, but I'll point it out anyway: For the vast majority of Japanese Buddhists, the religion has atrophied down to little more than paying a bunch of money to a temple when a relative dies and the number of people who could tell you much of anything about the teachings/practices of Buddhism outside of funerary rites is quite small.