So I've just installed English - United States via Site Administration -> Language -> Language Packs and purged all caches. Yet when I then go to set the Default Language under Language Settings it is not listed. I have even tried manually installing the the language files to the lang folder in my moodledata directory yet the same thing, it is not listed.
Any reason why this is happening?
EDIT: My mistake, I had only "en" set in Languages on language menu. Added "en_us" to setting and it is now listed.