If you take the metro to the Universitat stop, that's where those black marble ledges are. You'll see people skating flatground there and hanging around at any hour, day or night. MACBA's right down the street from there. Parallel, those manual pads with the metal coping, is a 5-10 minute skate from MACBA. Also, if you skate from Parallel to the bottom end of Las Ramblas (the party street) you'll see all kinds of random spots.
For the most part, the language barrier isn't really an issue because it's a tourist town, particularly with regards to skating. Everybody's from somewhere else, so English and Spanish are the most common languages since everybody's from Northern Europe or the US or other parts of Spain. I only speak English and Spanish and nobody ever tripped on me for not speaking Catalan.
There's a bar called Manolo off of Las Ramblas that's the 'skater bar.' Kind of a cool novelty stop, you might see some wasted American pros and they show skate vids there. Careful of hookers feeling you up on Las Ramblas or any random dude who comes up to you talking some bullshit-sounding gibberish language that isn't Spanish or English. Turn your back for a half a second and your wallet, cell phone, and backpack will be gone. Just give them a 'No, gracias' and bounce. This can happen at MACBA or Sants as well.
As far as tourist attractions that are totally worth it, Park Guell is the shit.