We have file a bug to report the problem:
6477533 P2 java_deploym/update jupdate dialog renders messages as boxes on Japanese Win2K

But this is still being evaluated by deployment team and the fix may not be ready until 5.0u11.

What we would like to do now is to undo the translation on Japanese resource file to revert back to the previous state (untranslated dialog instead of garbled dialog).