My /usr/share/locale is full of MESSAGES.po and other files for languages that I did not include in the LINGUAS variable. I use the 'nls' USE variable to get localisations in Italian, German and some English dialects: the languages specified in the LINGUAS variable. Still many packages install unwanted l10n like, for example, Novergian (sane-backends), Ukrainian (gimp, util-linux) or even the xx debug language (kaffeine).
*** This bug has been marked as a duplicate of 111899 ***