Americans may be ethnocentric, but not to the point where the reading public can't enjoy a book about someone from a different country! However, let's consider the opposite view. The movie High Fidelity successfully changed the British characters and setting to American ones. But that was because the American director, screenwriter and cast wanted to integrate it with their own experiences. In this case, I'm assuming you know Spain better than America, so your original version will probably be more authentic --unless you have lived in America for an extended period of time.
On the other hand, it's also often argued that the audience needs a character they can identify with, usually construed as straight, white, male, American and middle-class. When taken to its extreme, you get the phenomenon of "whitewashing", such as in the movie 21 where the true story of a group of Asian-American casino-breakers was dramatized with an almost all-white cast. This approach has been increasingly criticized in modern times, however, and the success of movies such as Moonlight or Crouching Tiger, Hidden Dragon shows that it may say more about prejudices than actual marketability.
In sum, I would make this change only if it actually improves the book in some way. If part of the book's arc is that you take a very ordinary, unexceptional character, and place them in exotic circumstances, then I could see a good argument to change your character to match the origins of the probable readers. But even in this case, I would use caution. Unless you know America from the inside out, your character is likely to come across as inauthentic in ways that wouldn't be noticed by the Spanish audience, but that will stick out like a sore thumb to the American reader (or publisher).