Sorry that you haven't received an answer for this question. I suspect that a bit of Googling about the temperature extremes in airplane luggage compartments might help you find this out. The airlines are not going to know the safe parameters for storage of PD solution--but they should know how hot or cold the luggage compartment beneath the plane is likely to get.
Not sure why anyone would want to take more than a bag or two of PD solution to Spain though, anyway. PD solution is available around the world. It would make more sense to arrange to pick some up in Spain than to ship water across the world.