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https://www.templatemonster.com/help/magento-how-to-set-default-language-and-currency.html#gref

1) go to Manage Stores and create a spanish store
see  http://i.imgur.com/jyBfDRb.png
this wont do anything yet , its just in case we need a spanish store that is 
going to have its own static blocks , pages etc in spanish

2)  go to :
/app/locale/ 
and upload the spanish package

3) go to system configuration > General > Locale Options
and select Spanish language

this will load the labels and text in all forms globaly to the selected language,
Note this will do that globaly , doesnt matter is store name is in english,
the stores that are created only affect if you duplicate the same content but in a different language