At Cuatro Vientos, ATC is in Spanish. However, if you call them up for taxi in English, they will reply in English. When they call your aircraft they will call you in English. If there are traffic matters that concern you, they will call you in English.
Having said all that, they do sometimes forget and the instructor can speak both English and Spanish and catches on. However, if you don't reply to their Spanish instruction, they then repeat it in English. The language barrier was never a problem for me doing my ME-IR. Madrid Control are even better, the majority of ATC is in English.