Installing and upgrading help

Dear all:

We are in the process to prepare our upgrade from moodle 1.5.4+ to moodle 1.8+. Although most of the courses on our production site are in English, we do have several Japanese and Chinese courses which are using customized language packs in 1.5.4+ (e.g. en_ja_utf8). Our initial testing (backup and restore the course into a clean 1.5.4+ installation, then upgrade this moodle installation to 1.6.5+) did not work. I did some search but couldn't find one discussion thread or any moodle docs that explain how to handle this kind of upgrade with customized language packs, could anyone please point out some solutions? Thanks!