CJKOS

To show Japanese Language, J-OS is not the only way to make it.
There are several ways to Japanize, CJKOS is the one of the famous application to localize.

CJKOS can show not only Japanese language, it can show Chinese, Korean language.
(but it cannot show these 3 languages in one screen.)

Just download from the website,
There are screen for setting but for the first-time user, it looks to be complicated.
Of course, it has manual, but I will introduce my treo600 settings to show Japanese as below.

CJKOS can run almost any palm devises.
But for Japanese users, there are 2 poor points.

1:CJKOS can show you any types of Japanese, but you cannot input Kanji with CJKOS software-keyboard. It can supprt inpout Hiragana, Katakata only. (So that is why, no check on Enable CJK Keyboard,)
2:CJKOS pre-installed Japanes fonts looks no good.Thickness of the letter is not same for each letters.

To improve these 2 weak points,
1:You can use other IME application instead of CJKOS's.(e.g. J-OS IME, PoBox, ATOK etc.)
2:You can use other type of Japanese fonts from this website.

As a result, you can read Japanese with your non Japanese version of palm devices.