Well, I think that Barcelona might be the 2nd and not the 1st. Athletic Bilbao is probably #1. First, they are Basque and the differences between the Basques and the "ordinary Spain" is much greater than Cataluna. Second, Athletic Bilbao have a policy - they field only Basque players. Not only they have no Catalan or Castillan (I must have misspelled that) players, you will not find "regular" foreign players in their squad, not matter if they're African, European or from Out of Space.
If I recall correctly, after a couple of difficult years, the management conducted a poll, asking the supporters what do they prefer, and the results said that they rather play on the 4th division than field non-Basque players... Amazing.
And more astonishing is the fact that despite this policy - they have never suffered relegation.