The main languages of Catalonia are Catalan, Spanish and Aranese (an Occitan dialect), and the three are official, but the Catalan is considered the own and main language by the Catalan laws.
Catalan (Català) is spoken in Catalonia, Balearic Islands, most of Valencia region, a strip in Aragon, Andorra, Alguer-Alghero (little city of Sardinia, Italy) and Roussillon (an area in the south of France sometimes called Catalunya Nord that corresponds roughly with the department of Pyrénées-Orientales). It is a Romance language and many words are recognizable to those who speak French, Italian, Spanish, or Portuguese. Locals really appreciate it if you try to say some words in Catalan while you are in Catalonia, and phrases such as "Bon dia" (Good morning) and "Adéu" (Goodbye) are heard even when people then go on to speak Spanish. Catalan is currently the first language of 35% of Catalonia's population (but is also the everyday language of 47%), and 93% of Catalans understand it. In rural areas Catalan is used almost exclusively, and while people there have no trouble with Spanish, they rarely use it except with outsiders.
Visitors who speak Spanish should note that in Catalonia the term "castellano" (Catalan Castellà) is used rather than "español" (Spanish). Most people in Catalonia are bilingual in Catalan and Spanish, since they are required to learn both languages in school. Replying in Catalan to a question asked in Spanish is the polite way to inform the other party that the conversation may continue in Catalan if desired. If you do not speak Catalan, just politely say that you do not understand, and you will be without a problem in most cases. Spanish is the first language of 45% of Catalonia's population and 99% of Catalans speak the language fluently.
Aranese is a minority language and only spoken by a couple of thousand people in the Aranese Valley in northwest Catalonia, but is also official.
As in the rest of Spain, English is not widely spoken even if is taught in all schools, though staff working at major hotels and the main tourist attractions usually know enough English to communicate. French is widely spoken in the areas near the border with France.