Most people speak it fluently, but Barcelona is the capital of Catalunya, and the main language there is Catalan (Català). It is similar to old Medieval southern French (Langue d‘Oc).
When the Arabs, or „Moors,“ invaded Spain in the 8th century, and occupied the center of the Iberian peninsula, their influence on the „vulgar Latin“ still spoken there was immense. But for the most part, they did not occupy the coasts. The Arabic-influenced language of central Spain (Andalucia/Al Andalus and the two main parts of Castilla) ended up as Castilian (castellano), which is often referred to as „Spanish.“ The western coast developed its own language, which became known as Portuguese. The central northern coast spoke Euskera, also known as Basque. The east, all the way down to Valencia, spoke what became known as Catalan, versions of which are also spoken in the Balearic Islands and Andorra. Catalunya and its language kept its close cultural ties to southern France, and even a tiny Catalan-speaking enclave in Sardinia. Written texts in Catalan predate the first written texts in Castilian, or „Spanish,“ by about a century. During the Spanish Civil War of 1936-1939, the Basques and the Catalans backed the Republic, and opposed the Fascists of Franco. When Franco won, he tried to eradicate both Basque and Catalan. He prohibited media (then mostly print and radio) in those languages, and prohibited schools to be taught in those languages. He sent Basque and Catalan school teachers to the Castilian-speaking South, and brought in teachers who only understood Castilian. With Basque, which is extremely complicated to learn, he was largely successful, but not so with Catalunya. There, the language and culture survived, and thrives today.
If your daughter will be there for a while, she could do worse than taking courses in Catalan. For a foreigner to learn Catalan, especially since one CAN get by without knowing it, this again sends the signal, „I know where I am, I know who you are, and I respect your culture.“ When I lived there, I lived with a Catalan family, so I learned to speak it well enough to sometimes be taken for a native. When North Americans learn it, it does our image more good than if we were to put Trump in a bathysphere and send him on a one way trip to the Mariana Trench.
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