If your Seven Ultimate install media is not Japanese, you must first install it in an available language (normally just one, some install medias can have a language choice) and then install a Japanese language pack.
An alternative that might work is to find a Japanese Seven Ultimate install media or ISO image, install it and activate with the Seven Ultimate key you have for your non-Japanese version. I have installed for instance a Finnish Seven using key from an English version I did not need, but also had problems when trying to Install an English version using key from a German Seven. Still today I can not understand the logics of this, although officially Windows 7 keys are edition but not language dependent.
Kari