Hi there and welcome!
I will give my opinion as I think I have had a pretty good experience in this realm.
I am from the US and I live in Barcelona, and I am here pushing 4 years now. It is a fantastic, international and cosmopolitan city, one of the best places to live in Europe. A misconception about Barcelona is that Catalan is the main language. Catalan and Spanish have equal levels of importance and use here in BCN, with all the people that speak Catalan also speaking Spanish. Catalan is definitely not necessary to learn here, but if you are here long enough it will sink in, and it doesn’t hurt to know!
Something I have a strong viewpoint on is coming to Spain to learn the language. Although I love it here, and have had a fantastic experience, I do not necessarily believe it is the best place to learn the language. Although the “mother land” of the spanish language, there is much more slang and accents, as well as etiquette differences here than is most other spanish speaking countries making it more challenging to learn. But what degree of challenging mainly depends on the region.
I have done a lot of travelling all over Spain, and then very much in Central and South America as well and have seen many variances in the language. I think if you are not dead set on coming to Spain for other reasons apart from learning the language you should consider some other options as well. Although some may not agree, from my personal experience with many people and time spent in the country, I have found Colombia to be the country where Spanish is spoken in its most clear and proper form, allowing for much easier comprehension from someone intending to learn.
Those are my thoughts, hope they could be of help. Best of luck!