I'm one of the original contributors for Japanese language files. I've updated
sitemap-ja.mo for version 3.1.4:
FYI, these Japanese files are no longer required for the latest version of Japanese WordPress package.
These character sets are no longer supported, but if you think you should keep them for backward compat that's fine. If you no longer support older installs (pre October 2010), then you can remove these files.