ja_JP.PCK and ja_JP.UTF-8 are displayed on some Locale Selection panels.
They are the following:
build38.0/sparc-S2/ml_cd_images_sparc/comp_cd_image_sparc/installer
build38.0/sparc-S2/ml_web_images_sparc/J2SDK_1.4.2/installer
build38.0/intel-S2/ml_cd_images_i386/cd_image_i386/installer
build38.0/intel-S2/ml_web_images_i386/J2SDK_1.4.2/installer

They are the installers that include J2SDK.
It seems that the cause is that pkginfo of SUNWj3jmp (a package for J2SDK Japanese man pages) has
SUNW_LOC=ja,ja_JP.PCK,ja_JP.UTF-8

However, I wonder if we can modify this pkginfo.