Hey,
there isn't 'General' forum, so writing in this one instead.
I'm using WordPress MU (which comes in English as a default language). Standard abbreviation for Jap. language in WP is 'ja' not 'jp'. Furthermore in repository the language files are 'ja.mo' and 'ja.po' (not like ja_JA nor jp_JP format).
To sum up, to make it work, WP and WassUp have to have the same translation filename (or at least the base - before '_' sign).
If it's working with Japanese WP install, then no problem. But certainly, I found some time ago, that correct Jap. abbr. in WP is 'ja' [which sounds a litte bit odd to me] - then it might be a bug.
Anyway, nice job with the plugin.
Thanks,
A.