Ticket #29456: wplang-test.php

<?php
if ( !is_multisite() ) {
        if ( defined( 'WPLANG' ) && ( '' !== WPLANG ) )
                add_filter( 'locale', create_function( '', 'return WPLANG;' ) );
        elseif ( defined( 'WPLANG' ) )
                add_filter( 'locale', create_function( '', 'return "en_US";' ) );
}