Non è questione di GET e di POST: semplicemente non puoi richiedere più di 500 caratteri alla volta, come è specificato nei "Termini di utilizzo del servizio":

You agree that when using the Service, You will not, and will not permit your end users or other third parties to:

* incorporate Google Results as the primary content on your website or page
* display business listings Search Results from the Google Maps service on any website which has the primary purpose of making available residential or business address listings or telephone directory listings;
* submit any request exceeding 500 characters in length, to the translation components of the Service;
Per i dettagli guarda qui: http://code.google.com/apis/ajaxlanguage/terms.html