Comment 10 for bug 666565

Referring to gettext document, which might be not really a standard but shows their attitude about locale, they give .UTF-8 as example, but not mentioning .utf8 at all.
http://www.gnu.org/software/hello/manual/gettext/Locale-Names.html#Locale-Names

I didn't do detailed research, but in some other major distribution they use .UTF-8 in their official documentations, e.g. http://www.gentoo.org/doc/en/utf-8.xml