It's hard to say really - there are so many witnesses anyway that the territory (Esp in the cities) is overworked , people know full well about the witnesses and what they believe
There are two things to consider here though....
First - with language "normalisation" legislation, everything in life must be equally availiable in spanish and catalan. So they are slowly releasing more and more literature that is in day to day use by brothers - meeting material etc. For instance, in the KH's they were legally obliged to put up catalan years texts and signs.
second - a lot of the brothers are very proud of being catalan, so much so that many prefer to go to english and other foreign language groups rather than speak spanish, and many older ones do not read spanish very well. The nationalism here is pride in the language (not pride in the state) so each region is proud of their own language (unfortunately where I live, that pride manifests itself in ETA terrorism). The difference is between basque and catalan - with basque it has become a political issue - in catalunya it is purely linguistic.