Spain is made up of a number of regions, many of which see themselves as separate from "Spain." Among others, Analusia, Catalunya, and Euskadi (Basque Country).
Post-Franco Spain has managed to give a lot of autonomy to these regions that had their individual cultures and languages repressed by Franco's government, and some think that federalism or even separatism is on the horizon.
In Catalunya, for instance, the Catalan language has regained primacy in daily life, though everyone also speaks Castellano, or Castillian, which most in the U.S. refer to as "Spanish". Catala is about 80% Castellano and 20% French. All signs in Barcelona are in Catalan and Castellano.
It is not unusual to find graffiti like this in many regions of Spain. We saw it in Catalunya, too.