What is your output (CHM, WebHelp, etc.)? Is the error
happening when you're working in RoboHelp, or is it when you're
trying to view the index in your generated help output?
The first thing I would try is opening up both the Japanese
HHK and the English HHK and compare the code. Make sure the
Japanese has all code intact, including punctuation. It's possible
that the translation process munged the code, and one missing
quotation mark or semicolon could break the whole deal.
If that's not the problem, try making sure your HHK is saved
in UTF-8 format. I would think that this would affect how the
characters display (whether you get Japanese vs. garbage
characters) rather than cause errors, but it doesn't hurt to check.
To make sure it's in UTF-8, open the HHK in Notepad, then go to
File > Save As and save it with UTF-8 for the encoding.
I had a Japanese index working in RH6, but it was with
FlashHelp, which has some unique factors affecting how double-byte
characters display.
Hope this helps,
Ben