the japanese.php language file included with geeklog is encoded in EUC_JP, but most input input as straight UTF-8 unicode.
save the language/japanese.php file in unicode with some unicode editor (notepad in Windows 2000 or XP works, as does Microsoft Word 2000, and many more), or get a modified version from my site here:
http://www.localsteve.cc/files/utf8-japanese.php
copy this file into the language directory and when you choose a language, select utf8-japanese instead.
now change the default charset variable $_CONF['default_charset'] in config.php to 'utf-8'. also make sure the following line is the public_html/layout/Yahoo/header.thtml file:
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset={charset}">
hope this helps.
steVe