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<?php
//make sure to change the domain from default to the actuall domain and it_IT to the language key you want
function _change_mofile( $mofile, $domain ) {
  if ( $domain == 'default')
    return substr($mofile, 0, -8).'it_IT.mo';
  return $mofile;
}

add_filter( 'load_textdomain_mofile', '_change_mofile', null, 2 );