I am not sure anyone is saying that Japan has become a multicultural society, and that is certainly a very different thing from saying Japan and Japanese are very diverse, which is true. I would add that the figure that the "foreign" population of Japan is less than 2% is obviously an under under counting given that many people counted as "Japanese" are in fact multi-cultural and hyphenated Japanese (i.e. Korean-Japanese, American-Japanese, etc.). In fact, the failure of the Japanese government and its statisticians to recognized hyphenated Japanese is probably the biggest shortcoming of Japan as a multi-cultural society. However, the definition of multicultural used in the last two messages seems to be more an ideal type than an empirical description of any actual society (e.g. how many Americans think President Obama is not really an American?).

Of course, your view of multiculturalism depends on the measure you use.
Americans often like to focus on one's birth. I remember a Tokyo based US academic comparing the initial Hatoyama cabinet with the Obama cabinet and finding the Hatoyama cabinet to be lacking in this respect. In terms of race and ethnicity, the Hatoyama cabinet was certainly far less diverse (although note a hyphenated Japanese, Renho, did subsequently join the cabinet). However, if we look at the nationality of academic degrees, a different picture emerges. After all, Hatoyama has a phd from Stanford, whereas I am reasonably sure no one in Obama's cabinet held any academic degree from a non-US university (I remember when Bill Clinton was considered exotic because he had briefly been an exchange student at Oxford).

During the several years I spent teaching at Japanese universities, I found that in the social sciences and humanities at least, Japanese academia has been remarkably open to foreign Phds. By contrast, US academia, at least in the social sciences and humanities, is almost entirely closed to foreign phds.
I say this as someone who has all of my academic degrees from American universities, but during my time teaching in Japan I met a number of very talented American academics who were locked out of teaching positions in the US simply because they had foreign (i.e. Japanese) phds.
Multiculturalism at the ideational level would be stronger in the US if its universities pursued greater diversity in their hiring policies.

Paul Midford