It is possible to change language for navigation part to English. Audio options and other menus will still remain in Japanese but navigation menu will change completely to English. This can be done by accessing hidden menu in the navigation system.
As for the Navigation CD, it is located behind the panel under the screen. this panel can easily be removed and navigation disk can be change, am not sure if any navigation disk for PK is available though.
I have changed language and Navigation disk for a 2004 mark X myself and it worked perfectly here in NZ.
Also need some help with Mark X navigation myself, I just purchased a Mark X 2006 facelifted model 2 days ago and the navigation disk is not located behind that panel. Does anyone know where the navigation disk is located for facelifted Mark X.