In Barcelona and Catalonia generally, street signs and other information are given in both Catalan and Castilian Spanish. It is the official language and is taught in schools and you will hear it on TV and radio.
As for learning it: Strictly speaking you can manage without it, as all Catalans speak Castilian. However, if you are living and working in any place you should try and learn at least some of the language; not only is it basic politeness, but it make people much more willing to accept you. In fact, in some jobs not speaking Catalan could be a real disadvantage or even prevent you from getting work.
One advantage of learning Catalan is that, unlike say Welsh which is totally different from English, it is close to Spanish and, if you've learned Spanish you shouldn't find it too hard at least to pick some basics in Catalan - so you could get a lot of results for not too much effort.