Learning Catalan is as important as Spanish if you are ONLY going to Barcelona. It's their first language, taught in all the schools, a year before they are taught English and 2 years before they are taught Spanish.
When I got off the metro by myself with all my luggage, I was supposed to meet at Paral-lel, and I just asked a bystander, "Donde esta el parque de patineta?" Which after a few confused faces being exchanged between the two of us, we started playing an impromptu game of charades and pictionary until she figured out I meant "skate park". Turned out it was across the street and about 100 feet away. I'd definitely recommend Catalan.
They said don't go out after 10 PM because you will just sleep in until 3 PM the next day. That city does not sleep. I didn't have any experiences that made me think it was dangerous to be outside, in fact I skated from Winkle's to MACBA on the first night at around 3 AM in my pajamas. There were easily 100 people skating, drinking and just hanging out. I could tell you a million stories about every day and night there, none of them were sketchy at all. Use common sense like you would in any city environment, and you shouldn't have any trouble at all. The city is so skateboard accessible it's stupid. You'll stumble upon so many perfect spots if you just roam. Go! Explore! Have fun!