French dictionary in Pages?

Pages uses the systems spell check. If you specifically want to use French and not trust the system to Automatically recognise the language use Spelling and Grammar in the edit menu. You can change the language to a preferred one

ashlander wrote:

Is there an easy way to find the definition of words in a French Pages document? Is there a French dictionary?

Yes, Apple provides a French-French reference dictionary. You need to activate it in Dictionary.app > Preferences.

If you are looking for translation, Apple does not provide that, you have to use Google Translate or something similar.