This guide assumes two things: you have Asian Language Packs installed and know how to navigate to your Control Panel. With these two, you can proceed on with the guide.
First, navigate to your “Control Panel“. You will be greeted with a plethora of options. The one we’re looking for is named “Regional and Language Options” which is the same exact option you used to install the language packs.
Click it and go to the last tab which is the “Advanced” tab. Under the heading “Language for non-Unicode Programs“, find and select “Japanese” or whatever language you plan on using for your games/programs. Click “Ok“.
You will be asked to either use the preexisting files on your hard drive or copy them from a CD-ROM. If you followed the guide, you shouldn’t need to. Click “Ok“. You will be prompted to restart. Do it and be done.
Alternative Method: AppLocale
This method is different from the first only in that you are not required to reboot your computer. However, you are required to manually apply this to every and all games/programs you wish to run in that language.
Download AppLocale here.
For those that don’t know Japanese at all, you will have to select “nihongo” or “日本語” when asked for the language. Here is a screenshot:
Sometimes, to be safe, I add this setting:
This can be reached by the same “Regional and Language Options” from the “Control Panel“. Instead of going to the last tab, go to the middle one that reads “Languages” tab. Click “Details…” and then go to the “Advanced” tab. Under the “Compatibility Configuration” heading, check “Extend support of advanced text services to all programs“. Click “Apply” and then “Ok” to accept the changes.