gettext convention is: a) unset LINGUAS => install all languages b) empty LINGUAS => install none The problem is, when we have any linguas_* flag in IUSE (e.g. when additional dependencies are needed), then portage will always set LINGUAS variable. In case none is enabled (which could be a) or b) ), this will result in: LINGUAS="" thus possibly overwriting the users choice of an _unset_ LINGUAS. This behavior is not present when IUSE does not contain any linguas_*.
This is fixed in git: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=207554dd7d2796f1fa4da41725154d17048a194d
This is fixed in 2.1.11.53 and 2.2.0_alpha164.