shin: I've found an old copy of that locales file from googling however the package is seriously broken and there's no hinting as to whether or not will this be updated or not.

Link to file: http://momoyan0306.web.fc2.com/n900/...5-r294_all.deb

Installation goes fine on PR1.3, however you won't get ja_JP in the Settings > Language & region > Device language. I have dug deeper via comparing to the unofficial n900-extras-locales-zhtw from extras-devel and it seems like the file structures were either outdated or quite different. In addition to all that, it seems PR1.3 has a locale-archive for all the locales to be compiled into one big archive hence the lack of individual files inside each of the /usr/lib/locale/<lang> respectively.

I have made some small hacks with minimal success. The first attempt was to compile locale-archive to contain ja_JP as localesdef --list will show there's no ja_JP entry.
This command then adds ja_JP locale from /usr/lib/locale/ja_JP/* into the locales-archive after a few moments of giving the device the time to remake the archive.

I then backed up /opt/maemo/usr/share/locale/ja/LC_MESSAGES before proceeding to copy the contents of /usr/share/locale/ja/LC_MESSAGES/* to /opt/maemo/usr/share/locale/ja/
After rebooting I was able to see the ja_JP locale written of course in kanji but sadly after rebooting when using ja_JP has lead to only English locale for all the GUI portion. Funnily enough some of the commands in terminal did get shown up with portions of Japanese characters such as wget --help, ls -l, etc.

Inevitably this is now where I am stuck. There's Japanese words going through to the programs in terminal yet on the GUI side only the date formats and the days of the week is shown in Japanese.

Update: I have tried making duplicate directories under /usr/share/locale/ja to have /usr/share/locale/ja_JP as well as /opt/maemo/usr/share/locale/ja to have /opt/maemo/usr/share/locale/ja_JP. Again no success.