Setting LC_MESSAGES: prevent unparseable messages

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 àaire" 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.