Microsoft Outlook Express is the email and newsgroup component of Microsoft Internet Explorer.
Japanese capability is available in both the English and Japanese versions.
For that matter, you can download language modules for a huge variety of other languages as well, which will let you read and even compose mail and news in those languages. The main disadvantages of this program, as I see them, are the relative difficulty of viewing unread messages consecutively from the keyboard, the lack of configurability, lack of remote server operations, the storage format, and the obscurity of such basic operations as viewing the headers. There are other annoyances as well, including the way it handles quoted text when replying to a message. This program is recommended only for the very lazy. Build sorting rules from existing mail.