Any advice on that? Use of undefined constant assumed 'WPLANG' in.... /wp-content/plugin

Hi there,

just wondering if one of you could give us a hint how to deal with the following error message as we are not familiar at all with .php or any other programming knowledge.

Message is
Use of undefined constant WPLANG - assumed 'WPLANG' in .... v2/public_html/wp-content/plugins/directory/core/custompress/core/core.php on line 57

Appreciate your professional advice to this.

Thanks in advance
Prinz