custompress bug - use layerswp and multisite

Hello, I test your plug-in but I got this error

Notice: Use of undefined constant WPLANG - assumed 'WPLANG' in /hosting/www/artecon.cz/www2/aesite/wp-content/plugins/custompress/core/core.php on line 57

Notice: Array to string conversion in /hosting/www/artecon.cz/www2/aesite/wp-content/plugins/custompress/core/core.php on line 81

thanks