You mean like 'this way to the egress'?
Terminals and train platforms are designated by letters and numbers. IE Terminal 1 = T1.
Signs from the terminal to the metro are marked with a directional arrow and a metro symbol.
Washrooms are marked with symbols designating Men and Women.
Terminal boards are marked with port number or platform number beside the destination.
I guess whatever can't be marked by some sort of universal symbol would be in Spanish ... stuff like 'no smoking by royal decree #1234' is in Spanish only. Don't let the pile of butts under the sign throw you.
If you are worried about navigating do a map recce before you go. Wikipedia is a good resource if you can't find what you want on station or service provider websites.
Here are AENA and adif websites. AENA manages all airports in Spain. adif manages the train infrastructure including stations.
AENA
http://www.aena.es/csee/Satellite/HomeAena/en/Home.html
adif
http://www.adif.es/es_ES/index.shtml
Oh yeah, the ATMs allow several language options. The problem is the directions to getting to the English instructions might be in Spanish.