One you need a Unicode font. These use 16-bits (2 bytes) for each letter so instead of 256 characters you get 65,536. Unicode fonts contain most characters used in every language around the world including Japanese and Chinese.
Second is you need to change the codepage of the browser. The code needed to change this to Japanese is:
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=EUC-JP">
Place that in the head section of your pages. I am not sure if you need to specify a dialect or not because there are a couple of different forms of written Japanese, I think Kanji is most popular though.