Barcelona is located within the Catalonia. There are two official languages in this region: Catalan and Spanish. That is why most of the television, newspapers and information is in Catalan. Therefore, it can be useful for you to learn a little Catalan during or before your stay. Moreover, it will make your international mobility experience much more interesting.
Even though Catalan language is very used in Barcelona, with some notions of Spanish you can understand part of it.
If you are interested in learning some Catalan, the University of Barcelona and other institutions offer some courses you might like to consider. You can find some at the Linguistic Service of the University of Barcelona. You can as well find some online courses at Parla.cat or Intercat.
If you are willing to learn Spanish, you should know that the Hispanic Studies Institute of the University of Barcelona offers courses aimed at Erasmus students: Spanish as a Foreign Language.