This locale variable defines how tools should display their messages.
This is for example gonna change the yum message from "Nothing to do" to
"Rien a faire" in my case (french).

As the yum module parses that string in err, if the message is not
enforced in english this is gonna fail.

So this commits just enriches a bit more the code that's already written
for that enforcement.

This commit fixes issue #9635.