As the capital of Catalonia, Barcelona has two official languages. These are Catalan and Castilian, the latter which is often called just “Spanish”. Many Barcelona residents are not originally from Barcelona and are not Catalan speakers. Outside the city you’ll find many more people are native Catalan speakers.
Most street signs are in Catalan, and Catalans are naturally bilingual and used to switching between the two languages. If you speak Castilian, most Catalans will switch. They want to make it easy for you. However, speaking even a few words of Catalan will earn you a lot of respect as so few people make the effort.
You’ll see many Catalan flags (one of the oldest flag designs in use today, with four red stripes on a golden background). A variant adds a blue triangle with white star at the hoist, used since 1918 to signify independence for Catalonia. This version is in widespread use and you’ll also see on on car stickers, t-shirts and other designs.
There is massive support in the city for FC Barcelona, one of the best known Catalan institutions worldwide. Other Catalan traditions you might see in Barcelona include castellers, impressive human towers (example above); a local dance called the sardana; various musical styles, including reedy woodwind and drums; and processions that include gegants. If you’re in Barcelona on a feast day you may encounter these.