Of course I have stuff from Japan at home, I also have a lot from Malaysia Taiwan and of course China. I'm not saying that Japan is insignificant, but if their currency can be treated as world reserve why not yuan?
It's about how strong the economy of certain country is, its about how stable the economy is, and of course its about how significant the economy is (The economy of Japan is the third largest in the world by nominal GDP).
As for yuan, as long as the Chinese government maintains capital controls on the conversion of its currency it cannot be used as a reserve currency
http://en.wikipedia.org/wiki/Reserve_currency#Chinese_yuan