I lived there for 10 years. Catalan is a regional language and will help you in restaurants., local government offices etc.,.but not always absolutely necessary. Everyone is bilingual. It's good to be culturally sensitive and at least try to learn a few words and phrases to integrate. You'll gain some respect from the locals if you do. Maybe you like it and then decide to learn more. You'll find outside Barcelona, Catalan is more widely spoken.
I can understand a lot of the Catalan language due to living there however, I spoke Spanish prior to moving there and it just so happens that it's the more dominant language.
Be aware of the politics, but I wouldn't engage in exchanging opinions. People have their mind pretty much made up. It just gets messy when football, language and politics are the only topics at the table. Try to talk about other stuff is my recommendation.
You're bound to enjoy things there. Barcelona is a cool city.