There. A neutral term. If somewhat anachronistic.
I guess it's ok, because Wikipedia uses it as well (and the Spanish Wikipedia too for that matter).
http://es.wikipedia.org/wiki/Elecciones_generales_de_Espa%C3%B1a_de_2012Spain is a parliamentary democracy, and the title given to its prime minister is "President of the government". "Prime minister" is a British term; in many other countries they use something else formally, but then say PM informally. For that matter, official title of Putin is "Chairman of the Government", but I never heard him referred to as "Chairman Putin", not even in Russian.
Not the case in Spain though. Zapatero is indeed known as "el presidente" there, since president is used as a short form for president of the government.