Lighting – How to soften the clicking sound of a light switch

You say that, when you release the button, the contact opens again (resistance goes to infinity). But the light stays on, right? That means there must be some device which has a "memory" of the last action: you push a button, the light goes on and stays on, until you press the button again (or another button). I'm sure there is a device in the cabinet (Dutch: "schakelkast") which toggles its output on every pulse it receives, and which in this way controls the lamp.
The wire on the top right of the switch also hints in this direction; it's a signal wire, it's too thin to be a power wire. My hunch is that it carries the pulses to the actual switching device (I don't know what it's called).

I would not recommend this. Switches are tested to electrical safety standards which includes making sure they have adequate insulated properties that prevent you the user from getting a shock. They are also designed to drain a short circuit to the earth/ground by cross bonding metal components built inside the switch. Modifying a switch could invariably invalidate those safety features. Making it a hazard for you and anyone else using said switch.