How to Use Separate Languages for WordPress Admin and Website

Define one language for the WordPress admin section and another language for the website.
*Note – for this to work you must not define WPLANG in your wp-config.php file.

Instructions
Add this code to your functions.php file

<?php
   // setup one language for admin and the other for theme
   // must be called before load_theme_textdomain()
   function set_my_locale($locale) {
      $locale = ( is_admin() ) ? "en_US" : "it_IT";
      setlocale(LC_ALL, $local );
      return $locale;
   }
   add_filter( 'locale', 'set_my_locale' );
?>