I can see some advantages to having the game check for patches etc, but I would like to play offline.
I've done some playing around.
If you disconnect or disable your internet link it will, eventually, enable a play offine button which you can use to launch the game. You can click on the little ? button in the bottom right to have a log pop up showing connection attempts. It tries so many times then gives up and offers you the offline option. This should always be available.
I have watched the traffic that the game sends out prior to starting the game and see it sends a bunch of stuff to google - why I wonder.
Anyway, I have also found that you can skip the initial blue logon splash and go straight to the full-screen game intro by passing a commandline parameter to the game exe, like this
CitiesXL_Game.exe -devdisconnected
I use the -devdisconnected parameter as other investigations suggest that is a parameter, however, anything will do, -help or -crap for examples.
This doesn't achieve offline startup, but it does skip one stage and does stop whattever traffic the launcher is sending to google.
So, come on MC, patch the game so we can chose to play offline if we please.
