Board Thread:Support Requests - Designing Your Wiki/@comment-25503763-20130606211444/@comment-10688548-20130606214251

"You have new messages on ... wiki" is MediaWiki:Youhavenewmessagesmulti. Just edit that page (rather, the equivalent page on your wiki) to say whatever you want.

It actually says by default "You have new messages on $1". Don't worry about the $1, it's just a variable (like X or Y in your math class) which stands for whatever the name of the other wiki is.