WP Admin Locale

admin-locale.php

<?php
/*
Plugin Name: WP Admin locale 
Description: Force en_US locale in WP Admin
Plugin URI: https://gist.github.com/f8c2f69b6e9f545696f7.git
Author: WPTailor
Author URI: http://wptailor.com/
Version: 0.1
License: GPL2
*/

/*

    Copyright (C) 2015  WPTailor  info@wptailor.com

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2, as
    published by the Free Software Foundation.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/


defined('ABSPATH') or exit;

function force_admin_locale( $locale ) {
  /**
   * We don't filter locale on options-general.php
   * because it's used in `Site Language` select.
   * Bail if doing ajax also.
   */
  if ( false !== strpos( $_SERVER['REQUEST_URI'], '/wp-admin/options-general.php' ) || ( defined( 'DOING_AJAX' ) && DOING_AJAX ) || ! apply_filters( 'force_admin_locale', true ) )
    return $locale;

  if ( function_exists( 'is_admin' ) && is_admin() ) {
    $locale = apply_filters( 'force_admin_locale_code', 'en_US' );
  }

  return $locale;
}

add_filter('locale', 'force_admin_locale' );
add_filter('plugin_locale', 'force_admin_locale' );

composer.json

{
  "name": "wptailor/admin-locale",
  "description": "Force en_US locale in WP Admin",
  "type": "wordpress-plugin",
  "authors": [
    {
      "name": "WPTailor",
      "email": "info@wptailor.com"
    }
  ],
  "require": {
    "php": ">=5.3.2",
    "composer/installers": "~1.0"
  }
}