Just to back up what others have said, I speak fluent Spanish (as well as a few other languages), and I wouldn't dream of doing my RT in anything else but English.
As has been mentioned, even at uncontrolled airfields people will understand your calls and generally switch to English (which may or may not help, depending on their pronunciation skills
)
Where I agree that learning Spanish RT may come in useful is in helping with your own situational awareness by understanding what everyone else is saying on the frequency... how you would go about that I do not know though, as there doesn't seem to be a lot of standard phraseology being followed.
Lastly, as for someone's jab at the French above, I am sorry but I do not think it is deserved. In my experience, English RT in France has been at least as good and consistent, and probably better, than South of the Pyrenees. Yes, local pilots will mostly use the local language, so you will find yourself in the same situation as in Spain and many other places, but not worse. Plus, France seems quite a good place for VFR flying in general, so please do not go slagging them froggies off gratuitously
