As SpamCop is a service that I'm paying $30 a year for, I thought I'd mention this problem on this forum and see if someone could help. All I can do is describe my situation. As far as technical solutions, I would have to leave that up to whoever has designed the system or who is responsible for keeping it running. I really don't understand a lot of what you're talking about, Wazoo, although I appreciate the volunteer help.
As far as the "default" font/character-set the SpamCop webmail system is using, if I set my language preference to Japanese, the menus are translated into Japanese (Inbox, Empty Trash, etc.), but the default is still Western, so the Sender and Subject Line come out as garbage. In that case, I may as well leave my language preference as English, as it doesn't affect the problem.
But something seems to have changed for the better over the last couple of days. I am now able to view properly if I manually change the "Character Encoding" setting each time to, for example, ISO-2022-JP. The only problem now is that my Inbox defaults to Western (ISO-8859-1) (as I just mentioned above) and when I open a message it defaults again to Western, and when I open it in a new window "to display properly" it defaults again to Western. Each time I go to the "View" menu in my browser and if I change it to ISO-2022-JP, I can now view the Japanese properly. But I have to do this three times. Even when my setting is for Auto-Detect Japanese (which remains checked throughout). If this "default to Western" (or whatever) problem can be solved, I will be a very happy camper.