Hi,
I'm just discovering this amazing helpdesk tool and hope this answer will help you.
The sentence you're looking for is in the "index.php" file, line 28.
As it is not meant to be a multi-language helpdesk at first, there are no language file to edit in order to customize that kind of sentences, unless you use what this guys has started to do (a long time ago) : http://osticket.com/forums/showthread.php?t=4469&highlight=french(http://osticket.com/forums/showthread.php?t=4469&highlight=french)
With this you will easily be able to customize the sentences in a language file.