When trying to run wordpress in spanish, you have 2 options.
From A Clean Install
You can start with a clean install, and download the spanish version of wordpress over here: http://es.wordpress.org/
Or, From An Already Installed WordPress:
If you already have an english version of wordpress running, follow these steps to run it in spanish:
1) Download the spanish version of wordpress and find this folder: wp-content/languages/
2) Upload the entire languages folder to your running wordpress at: wp-content/
3) Then, locate wp-config.php file at root of your wordpress.
4) Around line 72 you will see:
change this to:
5) Upload new wp-config.php file to you server and you are done!
You can check these results at www.cosasfaciles.com