Then I think your k800i may have already had some form of Chinese MAIN/FS installed unless you switched out the firmwares yourself to something that you like.

But the only Sony Ericsson phones that have ability to input Japanese natively is the SOxxxiS series from NTT DoCoMo or AU KDDI in Japan. Other than that you can only view Japanese characters while having chinese firmware and if you have something in japanese already you can copy and paste the words around.