i18n: `LANGUAGE` variable and fallback chains

[rdsq]

Right now afaict i18n is basically $LANG or fallback pair, and it would be really nice for users of unusual languages to have longer fallback chains like on software that uses gettext with stuff like LANGUAGE=sv:de, so this is my feature request 👉👈

[electricsteve]

I would also like it to respect LC_MESSAGES variable, set in /etc/locale.conf most of the time. According to the arch man page on it, LC_MESSAGES is meant of messages in programs. I have my LANG set to nl_NL.UTF-8, because I would like my dates, numbers, etc. to be in my locale, while all other things, eg. UI to be in English.