/usr/lib/locale is not for general translations - those should be installed at /usr/share/locale. To make matters worse, Kerin notes that sys-apps/locale-gen currently destroys the non-glibc locales stored in /usr/lib/locale, which means: a) obviously the functionality if it could ever work is broken upon locale-gen being run; b) files installed by a package suddenly disappear which is impure.
https://www.portagefilelist.de/index.php?fs=%2Fusr%2Flib%2Flocale%2F*&unique=1 lists some infringers.