I would have thought that FLUENCY in English, not just a working knowledge, should be an absolute basic requirement for any ATC controller, no matter where they are?
There are plenty of people in spain who speak fluent english, and speak it very well, surely it is not that hard to get some of these working in ATC at major airports?
Or perhaps ATC in spain is not paid so well/ does not carry enough prestige in spain to attract such well educated people, who are perhaps more tempted into the lucrative worlds of translation and international business.