You will need to completely uninstall UTAU and delete all related files, such as the installer.
Follow these instructions to change your system locale to Japanese:
https://www.java.com/en/download/help/locale.html
Make sure the beta option for using UTF-8 is disabled. You do not need to change any other settings, like language or region. Your computer will stay in your preferred language.
After your locale is fixed and you've restarted your computer, you can redownload the installer and reinstall UTAU:
http://utau2008.xrea.jp/