How can I make the thesaurus work for spanish words?

I have been using pages for about 4 months now, and usually I use it in english. I am currently writing a document in spanish but I can't manage to make the thesaurus, dictionary and wikipedia work for spanish words. I really need this to find synonyms.

Any idea what do I need to do? I don't want to change the whole pages into spanish, nor I want the spanish spell check (I know how to do that), I only need the thesaurus.