On PHPRiot.com there's a new tutorial showing you how to use the Google Translate service to translate the text of your website into any language they support. They interface with it using a cURL connection and JSON messaging.
They talk about the input parameters you can give the service, the enforcement of query limits and how to handle the results that are returned. The response message is, by default, in JSON so a simple call to json_decode should be all that's needed. They've also included a sample class you can drop in and use for your translation needs (as well as sample usage code).