Quantum ESPRESSO allows to calculate saturation polarization with the help of Berry phase calculations, along X, Y or Z axes. How to calculate the polarization along face or body diagonals?
You need first to calculate the polarization along x-, then along y-directions. So the resultant of the 2 polarizations vectors will be the xy direction you looking for. I.e. P(xy) = sqrt {P(x)^2+(P(y)^2)}.