It's because the eclass installs "en" only by default, but some upstreams use "en_US" instead. This has affected a few packages so this should probably be addressed in the eclass.
This is fixed in CVS and git. http://git.overlays.gentoo.org/gitweb/?p=proj/kde.git;a=commit;h=8b044a866ca48bcfc3a6cd673a9f866c50e5e9d9