I lived in the heart of Barcelona for a year, 5 minutes from Placa Catalunya in Ciutat Vella, and have been to Paris a few times, so I can give you good information on both.
First, getting through customs in Barcelona is a fukkin joke compared to America, so no visa or anything needed if you're American and are staying less than 90 days. The best way to get into the city from the airport and vice versa isn't through aerobus, which I've used before, but instead there's a train from the airport (Terminal 2) that rides you into Sants station...which is one of the major metro train stations in BCN. It's like 10 Euros for 10 trips, so you're basically only paying a Euro to get into the city. From Sants you can ride the green line to Placa Espanya, and connect there to go from Espanya to Placa Catalunya (The Center of the city). I recommend you get used to riding the metro there, it's really easy and intuitive.
About the Catalan language part, yes...many people there speak Catalan as their primary language...but you're still in fukkin' Spain and everyone knows how to speak in Spanish (or castellano there), so don't worry about that. Most of the time when people find out you're American they'll try to speak English to you. Basically all you need to know how to say to order food is "hablas ingles?" and they'll speak English back to you. A lot of people there try to take English speakers as fukkin idiots though and try to rip you off, so careful about that. For example, me (lived there for a year) and my girl (actually from Barcelona) went into a little convenience store off of placa sant Jaume, and the dude heard us speaking English and charged us 10 euros for like 4 euros worth of stuff. When I brought it up and spoke to him in Spanish, he was like "I thought you guys were tourists because you spoke English...." and assumed we wouldn't know the difference because of the language barrier on the receipts. Needless to say I told him to keep his shyt and we walked out. So be careful. It's mainly middle eastern dudes who pull that kinda shyt. Had people throw extra shyt on my bill at restaurants too because they assumed I couldn't read the receipt.
As for shyt to do in the city, there's a bunch. I highly recommend The Sagrada Familia and going up into the towers there, The Cathedral is pretty cool and you can go on the roof, Montjuic and the park around it are a nice place to take walk with a girl, Barceloneta is cool but usually packed this time of year, so if you want to go to the beach get off at the Vila Olimpcia stop or Bogatell on the yellow line and it's a little less crowded. Walking through Ramblas, Passeig de Gracia, and ciutat vella is awesome also, so I def recommend that. For clubs hit up Otto Zutz, Razzmatazz, Apollo, Pacha, & Opium....they're a little pricey to get into (like 15 euros) but they come with a fat drink so it's straight if you stay. A cool place I like to go to is called Le Cyrano. It's a cheap af bar and they literally let you pour your own liquor into your drinks....lots of Americans down there also so it's straight.
There's a lot more to do there...but I wrote hella already just giving you some basics. If you wanna know more send me a message and enjoy your trip.
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