Hi Tsunnako,
To translate language to Japanese (or any other languages). Open the language files of T3 blank template and T3 plugin,
1. templates\t3_bs3_blank\language\en-GB (t3 bs3 blank template) or templates\t3_blank\language\en-GB (t3 blank template)
2. plugins\system\t3\language\en-GB
Next:
1. copy the languages files of T3 blank/T3 BS3 blank template to: language\en-GB
2. copy the languages files of T3 plugin to: administrator\language\en-GB
Now see how it changes in template manager.
When it's done, you can contribute your work for community. See more at:
http://www.joomlart.com/forums/showthread.php?95782-A-humble-request-to-all-Purity-III-users....
If you need any help, please let us know.
Gardner.