The normal English version of Windows Mobile is not capable of working with Japanese. Making it capable is quite a trick. Search the internet, and you'll find many schemes for doing so. The commercial software for enabling Japanese simply doesn't work very well--one program is extremely limited compared to the system on the native Japanese version of Windows Mobile, and the other just doesn't work. However, with a bit of hacking in the English OS, it's possible to add fonts and the very same input systems used in the Japanese version (in fact, if you request it, I can also add the input systems from the Chinese and/or Korean versions, too). The end result: an English X51V that works just as well in Japanese as the Japanese version. You may have seen my description of the English system I sold up until March 2008. It warned about the eccentricities and minor bugs of the system, and I recommended that customers get the Japanese version instead. None of that applies anymore. The English version is stable and reliable and it works beautifully.
The main reason I've moved up to the X51V is that the Japanese support I can add to the English OS is much, much better than I can do with the X50V. Also, because the memory is no longer dependent on battery power, if the battery does die you don't have to restore the system (actually a fairly minor issue), and it lasts longer on each battery charge (a bigger issue: battery life was the achilles' heel of the X50V). Minor advantages are better versions of the built-in software (MS Office, etc.) and better and easier Wi-Fi connectivity, especially with secure networks. I'd resisted using the X51V for a long time because its Windows Mobile 5 operating system was slow and horribly unreliable (many users found it unusable because of frequent crashes); however, once I was able to solve that problem and make the X51V as fast and reliable as the old X50V with its speedy, rock solid OS, I saw that it could be a better base for the English system than the X50V. (Another big factor: the supply of factory refurbed X50Vs was drying up at the same time Dell began refurbishing and selling more and more X51Vs [`course, I've got to buy in bulk to be able to offer a decent price]).
I thank the many posters at xda-developers, Aximsite, and Brighthand for resources and information. I didn't find that any of the resources posted there worked perfectly, but the amount of specialized knowledge on those sites is astounding, and with the help of little bits of info from a hundred different posters and using those resources as a starting point, in a few weeks of steep learning curves and trial and error, I was able to put together a system that works beautifully. Thanks especially to the hacker/poster known as Asukal, who provided the resources that made putting Japanese on an English Axim possible. Because so much of the groundwork for the improvements to the English OS system are due to these folks, I don't charge any extra for my work in making the OS work in Japanese--I just charge for the hardware and my work on the dictionary system.
I expect a lot of people will be asking exactly how I did it. I'm sorry, but the procedure is complicated and getting it wrong is a lot easier than getting it right, especially since people may have their Axims set up in various ways that may conflict with different parts of the procedure--if I don't have the unit in my hands, I can't be sure what will and won't work. If I do give DIY directions, I know from experience that I'll buried under requests for free tech support that I don't have time to offer, and that from time to time I'll get angry emails from people who managed to brick their expensive PDAs and blame me.
You might find a couple of different software packages designed to give an English PPC Japanese capabilities. The two I've found are Effy-Japanese (Eng/Jp) 4.0 by TimeSpacesystem Co. and CE-Star. Effy doesn't have a handwritten entry option, and it doesn't seem to offer anything that the system described above (or the native Japanese system) doesn't, so I haven't bothered to check it out (the free system is much better, so why spend money?). CE-Star does have a handwritten entry system, so I gave it a try. First off, there was a stupid mistake in the installers (at two points, it says "Click the NEXT button" or "Click the START button," but there was no NEXT or START button; customer service told me to just hit Return at those points--this worked in one version of the software but not in the other). Once I got it going, I found the handwritten entry system unusable. With the input system from the Japanese version of WM, described above, I was able to find the right kanji the first time I wrote it almost every time--it was stunningly good. With CE-Star, I'd have to write even simple kanji over and over again--sometimes it found the kanji I wanted on the fifth or sixth try, but most of the time it never did. Even worse, you can use a virtual keyboard to enter kana, but there's no way to then convert those kana to kanji (a standard feature in every other IME ever made, and absolutely essential). CE-Star is clearly made for use with Chinese--the Japanese components, no more than an afterthought, are useless (if the installer issue is typical of how sloppy their developers are, the Chinese system is probably just as bad). I've been told the latest version is better, but that the handwritten entry function still doesn't work well.