Actually the string "System policy prevents install file" is what you provide in the .policy file; see the docs
for details :-). So it's really a PackageKit string... I suck at naming and coming up with user-friendly language so I can't advice you on something better.
(also, you want to remember to the .policy.in file add it to po/POTFILES.in (need intltool>=0.36) to get it translated; see the PolicyKit-gnome repo for details).
It's also worth noting that you can configure your system for 'wheel' group access (like Mac OS X) when there is no root password on the system; that's documented too and it amounts to adding an element define_admin_auth (with the attribute group="wheel" or user="davidz|pat" or whatever) to /etc/PolicyKit/PolicyKit.conf and then you'll get a dialog like this where one of the administrative users (according to define_admin_auth) can be selected.
It's probably the right choice to avoid allowing people to keep the privilege (cf. this dialog) but if an admin wanted he could even do this on a per-user basis in /etc/PolicyKit/PolicyKit.conf using the match elements (again, refer to the man page). Conversely, you can also arrange that some users will never even get to see the dialog.
So there's a lot of options :-)