Internet Explorer and Outlook Express have supported multiple languages
since the days of Windows 95, if the Internet Explorer version was recent
enough.
The multilanguage pack for the OS is completely different. This allows
changing the language used by the OS itself for menus, help files, dialog
boxes, etc. This only runs on Windows 2000 or Windows XP Pro -- and first
you have to install a particular foreign version of the OS (English
language) because it can't be installed on top of the domestic version
(Japanese) or one of your domestic versions (guess which one). But this
doesn't affect applications, and doesn't affect Internet Explorer or Outlook
Express. Those programs still support whatever languages they were made to
support.