I should have been clear. As a spanish, we consider spanish anyone with spanish culture. We do not care that much about the ethnic group. I think that is different from North America, where the ethnic group determines a lot what kind of "american" you are.

That said, when I say "you will never be a japanese" I am not talking ethnic groups. I am saying that even if you speak japanese and try to be japanese, people in Japan will always see you as an outsider. This is not the case, for example, for a black african raised in Spain who speaks spanish perfectly. They just become a fully integrated part of Spain and noone even questions that as long as he adopted the language and culture.