A quick Google search found several threads on here about this (!) and several from bike forums inside and outside the UK.
http://www.eyeonspain.com/forums/posts-long-7319.aspx seemed the most useful, so start with that one,.
It appears you have to take the Spanish test for a bike over 125cc. My first thought would be to head for the coast, find a bike shop, and ask there about lessons / tests available in English.
Or search Facebook for motorbike groups in Spain - eg there's an expat bike group near me in Los Montes de Malaga, and people like that can give advice and perhaps provide coaching leading up to the test.