citynames are not really translated, but picked from a list of available names. In the same folder you found ca.tab, there should be files called citylist_##.tab, where ## is the language. There probably is no list for catalan there. Changing the language after a city was founded does not change it's name, so switching to english only changed the displayed indicator that there is no name. For your current map, I'm afraid you'd have to name them manually.
I attached a list of catalan districts blatantly stolen from wikipedia. Can't test right now (at work), but if you add that and start a new map/found a new city, you should get catalan names.