Some systems accept aliases like "en" or
"en_US", but some only accept the full specification
like "en_US.ISO_8859-1". On Unix systems you can use
this command to see what locales are supported:
On my Ubuntu machine, I get
$ locale -a
So maybe the problem is "utf8" vs. "UTF-8".
This option must be set before loading menus, switching on filetype
detection or syntax highlighting. Once the menus are defined setting
this option has no effect. But you could do this:
From the comments, you tried putting it at the top of your vimrc. Perhaps there is a system vimrc file that gets in the way. Check with
:scriptnames. If so, then add something like the suggested lines.