Hi there,
Native from Catalonia here. Specifically, from Barcelona.
First of all, let's talk about the language because I see many concerning posts in this thread. Anyone that tells you that you won't be able to speak Spanish in Barcelona is highly misinformed to say the least. I've worked with people from around the Spain without any issue, either for them or for me. No issue either when you visit the city, go to restaurants or anything like that.
What is more, my parents are from southern Spain and I can tell you that my mother still does not speak Catalan after 30 years, just because she does not want given that she feels more comfortable speaking Spanish.
Obviously, as with any other bilingual nation, people here feels more comfortable speaking either Spanish or Catalan, or both. In my case, being raised in Spanish and talking with about 80% of my friends in Catalan I feel comfortable with both. I've seen native people having trouble both with Catalan and Spanish, but you, as a foreign, most probably won't be able to realize that. Funny thing, I'm from a town 20 miles from Barcelona, and when I moved to Barcelona I was surprised how much people defaulted to Spanish.
What's not debatable is that when people speak one to another, they might do so using Catalan, so in those cases you might not be able to understand it - at least for the first half year, given that if you understand Spanish you will only need some time to train you ear to understand Catalan too.
Another point, answering other posts. In some cases, Spanish accent is really complicated. Depending where do you go, sometimes not even a native Spanish speaker will be able to fully understand everything (e.g. you go to Cadiz). The same might happen with Catalan if you go to Mallorca ;)
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Regarding the tech/jobs scene. Right now, Barcelona is living a bubble in the tech field. People saying that the salaries are low or tied to the cost of life are people who might have been living abroad for some years being unaware of this fact, at least in Barcelona's case.
What's happening in Barcelona is that many foreign companies are opening branches here because it's easy to attract talent from around Europe (good weather, great lifestyle, safety, beach, mountains/forests, etc) and college here are quite good too. As someone has stated, senior salaries for developers go around €50K, which is a LOT taking into account the cost of life (e.g. nice flat in Barcelona's downtown might be €800, daily lunch menus are €10 - first, second dish, desert). In general, educated people not in the tech field here might do about €30/40K and have a very good lifestyle.
There's a lot of foreign cybersecurity companies operating here, many dev startups too, big consultancy companies and, as I said, many other companies coming from abroad which make the salaries go higher and contributing to the bubble.
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Lifestyle is unbeatable here. I was in San Diego, CA for more than a year, so I know what I'm talking about :)
What most people coming from abroad miss is the opportunity to get the best from two worlds. Being able to work and experience Barcelona while living in a suburb where you can walk to the beach and bike/walk through the forests. That's something Madrid cannot give you.
One last thing, whoever tells you it's perfectly OK to go around with English only is highly misinformed too. Not everyone speaks English here. If you go around the streets trying to talk with people it's quite likely you might find people who don't talk English. Another story is in your workplace. Most likely people will ""talk"" English if you work in a tech job. But bear in mind, when I say talk I mean babble. You'll have trouble understanding them in the beginning, but I guess they will improve over time as you will with Spanish.