You can only purchase from a particular country's App Store if you have a registered credit card in that country and if that credit card is tied to a physical address in that country. So unless you have a credit card in Spain registered to an address in Spain, you can't buy from their App Store.
If you meet the criteria and you need to change the store that you are currently in....
Go to Settings>iTunes and App Stores>Apple ID
Tap your ID
Tap View Apple ID
Enter your password
Go to country/region in the popup window to change the store
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