If you own a Kindle Fire and you live in Spain then it will connect to Amazon’s Spanish web site for your purchases. It is the same if you have an Apple iPod, iPad or iPhone; your Apple account on iTunes will be in Spanish. The issue arises because you need to register a credit or debit card to make purchases and because your cards are tied to your address in Spain, the site you are taken to is in Spanish.
With the iTunes store, you can search for titles that you are familiar with but all the recommendations made on the site will be for Spanish ones. I imagine the same applies to the Kindle store. With hundreds of thousands of expat Brits living in Spain, you would think that Apple and Amazon would work out some way that you could have an account in Spain that was in your native language and offered you titles in that language.