Its really easy to switch back and forth from English to Japanese and vice-versa.
Step1: Just go to control panel and got to Date, time, languages, and regional options and click Regional and Language Options.
Step 2: After you should get a window labeled Regional and Language Options. There will be three tabs on the window, Regional options, Language, and Advanced.
Step 3: If you haven't already you 'll want to go to Language and click one of the two little boxes, you want the one that says install East Asian files. If you're lucky your computer will have the file you need to install those languages, if not you'll need to look for your operating disk, (Ex:Windows XP.)
Oh you're in luck, one of our other members posted the files needed to install the Japanese part of the East Asian files. Please follow all instructions in this link too.
https://www.fakku.net/viewtopic.php?t=15076
Step 4: Now it'll probably ask you to reset computer, please do so, and finally go back to Regional options, Language, and Advanced, and this time go to Advanced. There should be an option to change non-unicode programs to many different languages, pick Japanese and you'll be on your way.
Oh and to ease your fears changing non-unicode programs to Japanese barely does anything. The only program that got changed into Japanese for me was Apple software updates. Plus some english symbols changed but it wasn't really a big deal since I barely saw them