Will going to Barcelona on my year abroad be detrimental to speaking Spanish?

Catalan and Spanish coexist in both society and schools in Catalonia, but the PP are determined to make Spanish the more prominent of the two. Also, Barcelona in particular has a lot of Latin American immigrants so Spanish is widely spoken outside school as well. Most Catalans are bilingual so you are very unlikely to meet someone who only speaks Catalan, and although they may prefer speaking it, they are happy to speak Spanish to those who don't speak Catalan. Please do not miss this opportunity! Barcelona is an amazing city with amazing people. I couldn't have asked for a better experience.

Last semester I studied in Vic, a small city just outside of Barcelona, and I felt exactly the same way as you before I went. I was worried because I knew that it was a Catalan speaking place, and it isn't very touristy, so I was worried that people wouldn't want to speak Spanish to me, and I didn't want to offend anyone by speaking Spanish and not Catalan. However, I can honestly say that linguistically speaking, I got more out of my time abroad than some of my friends who went to other parts of Spain, because both my Spanish and my Catalan improved.  Everybody was happy to speak in Spanish, and I was made to feel very welcome. In cases where a local person initiated the conversation, it was nearly always in Catalan, but once I explained that my Catalan wasn't very good but I could speak Spanish, they changed straight way. I think it's quite important to make an effort with Catalan if you have the opportunity because it is really appreciated if you can say simple things like 'hello' 'please' and 'thank you.' I was only an hour away from Barcelona so I visited quite a lot, and I was never once made to feel under pressure or embarrassed for not speaking Catalan. I think the people in Barcelona are used to others not speaking Catalan, so people in shops etc. nearly always go straight in to Spanish anyway. 

I studied a bit of Catalan at university before I went so I was able to say basic things (but very badly!) but some of my friends who had never done any Catalan went to places like Barcelona and Girona, and they had no problems at all! I would say go for it because Barcelona is amazing, and I'm sure you will be made to feel very welcome! :)