How to invert x and y, but not z

After finishing my Rostock Delta Mini Pro, I notice that the X and Y movements are inverted as to the grid arrows in the Repetier Host control panel.

I tried to set invert_x_dir and invert_y_dir to "true" but than homing will scatter to the endstops continuously for some time
Only inverting one of those will speed a tower completely down, emergency stop needed.

So, how should it be done ?

Thomas