Force english on WP admin !

gistfile1.php

<?php
/*
  Plugin Name: Languages Admin
  Plugin URI: http://www.beapi.fr
  Description: Force English on WP admin
  Author: BeAPI
  Author URI: http://www.beapi.fr
  Version: 0.1
  Text Domain: bea-languages

  ----
  Copyright 2015 - BeAPI Team ([email protected])
  ----
 */

class Languages_Admin {
	public function __construct() {
		add_filter( 'locale', array( __CLASS__, 'locale' ), 9999999999999999 );
	}

	public static function locale( $locale ) {
		if ( is_admin() ) {
			$GLOBALS['locale'] = $locale = 'en_US';
		}

		return $locale;
	}
}

new Languages_Admin();