How to: Add additional languages to your WordPress installation – Multisite Multinetwork localization
Generally when working with WordPress all you had to do was to type an entry into the config php file to set the language you wished to use like so:
define (‘WPLANG’, ‘es_ES’);
With Multisite installations you can actually select from the Settings / General section the language for each individual blog. Keep in mind you have to have at least one language file in wp-content/languages folder for that option to appear. Out of the box that folder is not created and you have to download the language files either from a wordpress site for that particular country/language like es.wordpress.org and extract that folder into your installation.