I usually read messages in /var/log/portage/elog/summary.log to simply
warn me about "es es_ES" LINGUAS not being supported by that package.
That comes from eutils.eclass inside strip-linguas:
ewarn "Sorry, but ${PN} does not support the LINGUAS:" ${nols}

Is this warning really required, personally I know a lot of packages
that don't support spanish translations and I get no warning from them
(as they don't use strip-linguas). When I set LINGUAS variable in my
make.conf I assume a lot of packages won't support spanish translations,
and I see no point on being informed about that for some packages using
strip-linguas.

What do you think?