When posting Japanese comments (and presumably anything with 2-byte languages like Chinese, Korean, and Japanese) the "apply_filters" function which is called from the "comment_text()" function in the template_functions.php file produces totally mangled Japanese text.
You have to allow for the fact that two byte characters can be coming into your functions and not slice and chop strings apart on the assumption that they are single byte characters.
TEMPORARY SOLUTION FOR OTHERS EXPERIENCING MANGLED COMMENTS IN OTHER LANGUAGES:
In the template_functions.php file in the wp-includes folder, comment out the following line in the comment_text() function like this:
/* $comment_text = apply_filters('comment_text', $comment_text); */
I don't know what apply_filters does but it seems to work without it.