I don’t think you need to learn catalán. People in Cataluña speak Spanish as well so you should be fine speaking just Spanish.
The best way to learn a language is living in a country that speaks it but you could learn Spanish for a couple months before you go so you can at least say basic things and then go.
Spanish is a very useful language as a traveller. And remember Spanish is not the same everywhere, the Spanish we speak in Spain is not like the one spoken in Argentina. We do understand each other but the accent and words we use are not always the same so be flexible.
I learned Portuguese for 2 months before moving to Brazil, where I am right now. And I am learning more by living here cause I am forced to hear it all the time, read it and speak it. Brazilians don’t really speak foreign languages so it is a great country to practice Portuguese.
Why not housesitting? I never did it but there a lots of offers on FB.