As it currently stands, the locale.gen installed by sys-libs/glibc is very skeletal and does not list all locales which can be enabled for a given glibc version. Generate a list of locales which the glibc version supports in that file in order to ease non-en,ja,de,es,fa,fr,it users by showing what is available right in locale.gen. Uncomment/enable en_US.UTF-8 by default in order to prevent building all supported locales, as it can take a fair amount of time on weaker machines if the user does not edit it before doing the initial @world update.
the header of the file explicitly documents how to generate a locale and then says to check out /usr/share/i18n/SUPPORTED for the default set which should answer the vast majority of users' needs. generating the "full set" is unreasonable as you're talking about the combinatorial explosion of all the locales in /usr/share/i18n/locales/ (300+) and all the character maps in /usr/share/i18n/charmaps/ (200+). which is is 60k+ entries. as for only enabling en_US.UTF-8 by default, no. that is hostile to the majority of the world who do not follow US cultural norms and do not speak English.
