This week I had problems with changing the language of my multisite, for those who have the sample problem I wrote a quick guide:
On a normal WordPress installation you can change the language in your wp-config.php file. When I like to change the language to Dutch (by example) I define:
define ('WPLANG', 'nl_NL');
Make sure the language files (.mo) are located in your languages folder in wp-content.
But I recently tried this on a multisite, and nothing happened!