If you ever feel the need to install a language pack in magento it can be done by selecting the key from the magento commerce site(extensions) for the respective language pack. However the Spanish language key given (magento-community/Locale_Mage_community_es_MX) may not work hence you can use an alternative mentioned below:
magento-community/Locale_Mage_community_es_MX-1.0.19870.4.5

If the above method of installing a language pack fails then browse to the site mentioned below and download the language pack:

http://www.magentocommerce.com/langs

You will need to install it manually however.

The language can be configured from the configuration menu in the admin by setting the locale.