Have you checked in Fontbook to make sure any/all Korean fonts are enabled?

Don't suppose anything is still running Tiger...if so, you might want to compare the installed/enabled languages. You can check that Korean is even installed with the "International" Pref Panel. In the Language" tab, mouse over the list and the text tool tip will report the names of any that are using non-roman characters. 'Korean' seems to be just two characters in my standard install.