if (preg_match('/^\/wp\-admin/', $_SERVER['REQUEST_URI'])) {
    define('WPLANG', 'en-US');
} else {
    define('WPLANG', 'de-DE');
}

However, this fix left adminbar in de-DE, he saids.

To fix adminbar to be en-US while frontend html in de-DE, activate switch-lacale.php plugin hosted in gist and set define('SITE_LANG', 'de-DE'); in wp-config.php as frontend html lang.

It is an old post, so I dont know if it works now.
Hope this helps.