No, you cannot get a shock from 12 volts, so there is no need to switch off the mains.
However, do remember that the 12V bulb is connected to a transformer which converts the 220V mains into the 12V needed by the bulb. This means that lethal voltages are present at the other end of the transformer. This transformer may be installed right behind the bulb (inside the lamp fitting), or it may be some distance away inside the wall or ceiling.
Either way, it is very unlikely that you will come into contact with the mains voltage. However, do make sure the light is turned off at the switch. The main reason for turning it off is to ensure you don't short out the transformer when poking around with pliers, as that could possibly destroy it - but even then it would still keep the 220V away from you.