If you're running this under a Windows OS (I can't tell, since I don't skin my Windows anymore, hence I don't recognise if that's a theme or a different OS D:), you should either:
1. Manually change your system locale (and language settings) to Japan. This may require you to have your Windows CD as the files needed may not be on your computer already.
2. Use Microsoft Applocale to temporarily run the installer under Japanese settings.