The message is a security feature to make sure that the user’s desktop/login screen can’t be spoofed. As such, being changeable would defeat its purpose. However, adding a selectable message (including different languages) would be a worthwhile feature request…
To spoof something is to create a fake version and use it to fool the user into letting you do something that you shouldn’t. Thus I can spoof your email address to send something naughty to your boss and get you in trouble. Worse, about 100,000 jerks try to spoof E-Bay or Paypal every day to get people to “change” their password – but they end up just giving it to the bad guys without knowing because the site has been spoofed/faked.
That message is there to let the user know that Unity has taken over the screen so everything that appears is doing so under Unity’s control and not the OS’s. This means some less-than-honest person couldn’t use Unity to display a fake desktop with some error message and a Password box asking for the system password. “To continue this operation, the system must first be optimized. Enter the system password below to continue…” or something similar.
Spoof’ing rarely ever works on computer-savvy people but it fools so many of the more helpless users that a built-in safeguard is a respectable solution.
Having said that, adding a set of localized strings for use in different countries and/or languages is a really great idea.