Forum:Message notification customization

Does anyone know how to customize the message notification (for when someone writes on another user's talk page)? The normal says "You have new message(s) at [Wiki here]", but I saw the CSI Wiki had one that said, "You're needed on the 419 at [Wiki Here}", playing off its law-enforcement theme. How was this done?
 * Edit "MediaWiki:Youhavenewmessagesmulti". -- 03:42, June 28, 2010 (UTC)