Translate forum update notice.

Hi.
I want to translate the notice that the forum gives when we update it.
Let me explain: when we are updating the forum (yesterday I did it), if someone enters the web, a warning appears indicating that it is being updated.
The notice says: Update in progress ... blah blah blah.

I can not find how and where to translate that.

Nobody knows how to translate it or how to locate it to translate it?