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
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. www.gnu. org/software/ hello/manual/ gettext/ Locale- Names.html# Locale- Names
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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