What side of a European mail truck is the drivers side?

Oh, well, this actually depends on, if you count the UK as belonging to Europe or not ...

In countries, that drive on the right side of the road, the driver´s side is the left side .. in the UK its the right side, since they drive on the left.

I´m not aware that mail trucks differ from other vehicles in this.

Trucks that clean the roads, however, sometimes do. They have the steering wheel on the "wrong" side, so its easier for the driver to navigate around parked cars ...