hi. i work in BCN in one of those companies some people mentioned. some notes:

1. if your company is big and international you won't need to be super fluent in spanish. the city has plenty of expats so you spanish/catalan is as needed in barcelona as german is in munich (i lived in munich as well). you say you're functional, that's enough for now, you'll figure it out once you're here. if the company is "a startup" then it's a lottery, but most likely more than half of the people will be spanish speakers with some english fluency. they will all speak it but they'll often speak to each other. this is something that happens in my company on a daily basis.
2. salaries are an odd topic. i can't compare to the US because i have no idea what people do with money there, but in terms of an average techie lifestyle i can say that barcelona and london aren't very different on a net level. you might get (unlikely) 90k in london for a senior dev position, and this is factually not much different from 60k in barcelona since most of the fixed costs (rent, public transport) are half the price. 55-60k for a senior dev position is possible (trust me) but not in a startup, unless you manage to snitch a lead position, which might happen. i've seen people moving out of normal position (not entry, not senior) and becoming leads in startups.
3. the comparison was just with london but the same applies to paris. it probably doesn't apply to berlin since berlin has high-ish salaries (comparatively) and low rents, but it has its disadvantages.
4. even if 2. and 3. don't apply i.e. you still are worse off on a net scale (after rent, tax, etc.), barcelona has its perks over london and, for my experience, munich. these are entirely subjective but if having a terrace with a hammock you can float in december with a hoodie drinking cocktails sounds like a nice idea to you, then surely you can't do that in paris or london. food is also a plus, and – not great to say that out loud but – since the expat scene is a bit of a bubble and the locals have slightly different lifestyles you might easily find yourself in authentic gems (restaurants, bars) you don't need to queue for hours to get in.

having said that, here's a few cons:
* the tech scene isn't great. if you hate your new job you might have to relocate or search for something which is "alright" but not great. things are getting way better than even just 3 years ago but barcelona ain't no london.
* if you're looking for 100k barcelona isn't for you
* if you end up in an old-style spanish company (god forbid) you'll find out that the traditional mediterranean working culture is madness (compared to, say, germany or sweden). if you come from "strict schedules", "insane hierarchies", "little meritocracy" etc. then it's probably comparable. newer companies (spanish/catalan too) are much better so it's not a matter of genetics (obviously, just in case somebody thinks i'm a huge racist), rather, inefficient traditions
* there are virtually no parks compared to even paris
* house prices are very random and estate agents are a laugh. i found a very nice loft which was very close to a club. i told the guy "well i'd like to figure out how noisy this gets" and i'm pretty sure his idea of "figuring out" was renting it and spinning the wheel. if you come at the right moment you might find an 80sqm flat for 1200€. if you're unlucky you might pay 1900€.

i hope this is clear enough :)