The transmission fluid may have overheated and burned, especially if the car is overloaded or has been towing. It probably needs changing, but this needs to be done by dialysis at precisely the right temperature and pressure and I don't know how you will get a proper job done in Spain, even by a Volvo dealer. See what you can find on Google. If you are at the coast where there are a lot of expat Brits, you might find a Brit run garage, but the transmission fluid cannot simply be drained and replaced. It has to be pumped out and the fresh fluid pumped in by specialist kit.