AFAIK, some messages come from server, and the language for these ones is set server side, and some other messages come from cli itself.

For example, my server has started with -Duser.language=en, and cli run on a system in french. The first message is in french and the second one is in english :

[disconnected /] connect localhost:9998
The controller is not available at localhost:9998: java.net.ConnectException: JBAS012174: N'a pas pu connecter à remote://localhost:9998. La connexion a échoué: JBAS012174: N'a pas pu connecter à remote://localhost:9998. La connexion a échoué: Connection refused
[disconnected /] connect
[standalone@localhost:9999 /] :foo
{
    "outcome" => "failed",
    "failure-description" => "JBAS014884: No operation named 'foo' exists at address []",
    "rolled-back" => true
}

How can I configure CLI so that all the messages are in english ?