First of all if you want Spanish user locale to be displayed on your ip phone. You will need to download the spanish user locale from CCO voice download page for the version of Callmanager you have installed and running.
Once the Spanish locale has installed on all of the servers in your cluster Pub and subs. You will have the option to choose the user locale either option for spanish or english under device pool, device phone pages on CCMAdmin.
There is no button on the ip phone to change the locale display either from english to spanish or vice versa. As an Administrator you can change it directly from CCMAdmin phone page or the extension you wish to change locale. Or simply have the user login into ccmuser page with there password and have them change there locale for there phones.
Here is the link where you can download spanish locales.
http://www.cisco.com/kobayashi/sw-center/telephony/callmgr/locale-installer.shtml
Note: Make sure the user locale version you install on your callmanager matches with the main Callmanager release you have alread installed.
Cheers!
-Yavuz