In Spain you need to make a will leaving property to whoever you want to leave it to, wife, mistress/toyboy etc. There is a form of wording used by Spanish lawyers that effectively means that al your assets in Spain go to the main beneficiary, with the exception of specific property going to named persons. In other words, once you make such a will, any additional property that you acquire later will automatically also go to the main beneficiary, i.e. all your Spanish assets at time of death, whether acquired after the will was written or not.
You have to be sure that the will is written in this way, but as I recall this is the standard method of writing it.