Hi hiradouk400. Definitely a reasonable question, and happy to help you out with it. The translations for our plugin are all provided by users of the plugin who took the time to complete a translation file using POEdit and send us the files. Looking through our records, I see we have a fairly complete Japanese translation builded with the current release of TEC.
Switching the language, then, should be easy. You'll just want to open up your site's wp_config file and change the language to the appropriate Japanese language code (which you can find here: http://codex.wordpress.org/WordPress_in_Your_Language). Re-upload the file and your language change for the plugin should be reflected immediately.
Hope that helps. If you find the translation file lacking and would be interested in updating/completing it in exchange for a PRO license, hit us up (pro a/ tri.be) and we can totally make that happen. Similarly feel free to let me know if you have other questions here. Thanks for using the plugin!