Most tech startups work in English, specially the ones with international/remote workers. Besides that, the "working language" is Catalan, because Barcelona is a city in Catalunya.

Language isn't a problem, as long as you speak English. Outside work, you'll hear both Catalan and Spanish. Both are easy to learn and understand, and people do really make an effort to understand and be understood, so no problem there either.

Barcelona is a different city. Most of the europeans come to Barcelona because we're used to speak english, if you compare to other cities where people barely speaks spanish well. Barcelona is multicultural to a high level.

At my job we all speak english. And sometimes we speak mix english / spanish but its rare.