bchan, I use Vista Ultimate and although SOME games will work with the English regional setting, many games will not (as in the past). Changing it to Japanese does do odd things such as altering default fonts in some programs as well as switching the backslash into the Japanese Yen symbol. Nevertheless, it allows you to play while still retaining an English Windows interface. I don't think
downloading the Japanese MUI would help if you still retain the English regional setting, as Windows would probably
still attempt to read non-Unicode programs in English, hence returning gibberish. If anything, I'd think an entirely Japanese windows interface would make it less useful than just changing the region.
You can try experimenting for the betterment of mankind and getting back to us.