Pages does contain a Spanish localisation. Application language should automatically match the system language, so long as the localisation is present. Do you have your system set up in Spanish or is it in English (or another language?). Go to the Apple Menu () and choose System Preferences. In the 'International' pane, choose the 'Language' tab and drag Español to the top of the list. However, this will swap your entire system and all supported applications into Spanish. Or did you just want Pages to be in Spanish and the rest of the system to be another language?
If you want Spanish spell checking, Stephen's answer gives that answer too. Just change system language.
If you want your system to be in English but spell checking in Spanish in one document, you can change language in the Inspector > T > More and change Language to Español.
If you want to keep your system in English but to have every new document to spell check in Spanish only, that is possible as well playing with templates and styles, but it takes some fiddling around.
The original question is not particularly clear. I assumed the OP wanted to change the application language into Spanish rather than just the spell check function. In any case, changing the system language will change the application language and (as Magnus rightly points out) selecting one's own written text and changing it to Spanish in the inspector will make Pages spell-check it as Spanish.
Just as there are fiddly ways to keep the system in English yet have the default language in Pages as Spanish, there are also ways to change just the Pages application into Spanish while keeping the system in English. We'll need to hear more on what the original problem actually is.
Thanks for the help. The reason I need spell checking in Spanish is for my term papers in Spanish. I have been using Office:mac for the last year and always used Spanish spell-checking under Tools. I am new to using Pages, so I appreciate the information.