<?php
/*
Plugin Name: English Only Admin
Plugin URI: http://your-domain.com
Description: Force English (en_US) in the WordPress Admin
Version: 1.0
Author: You
Author URI: http://your-domain.com
Text Domain: englishonlyadmin
*/

// prevent direct access
if ( ! defined( 'WPINC' ) ) {
    die;
}

if ( ! function_exists( 'uniquePrefix_force_english_only_admin' ) ) {
    /**
     * Override locale for admin to force English (en_US).
     *
     * @param string $locale Current locale.
     *
     * @return string English (en_US) locale if in Admin, configured locale otherwise.
     */
    function uniquePrefix_force_english_only_admin( $locale ) {
        // detect when we are in the admin dashboard and force english
        if ( is_admin() ) {
            $locale = 'en_US';
        }

        return $locale;
    }

    add_filter( 'locale', 'uniquePrefix_force_english_only_admin', 1, 1 );
}