What does the world know about which languages various Google desktop
products are written in? We know that Google Maps, for instance, is
written with "Ajax" technologies. But how about the Google Toolbar, or
Gmail, or Picassa? Are there user groups for these programs? What, if
anything, has the company said about how they are made?

This may help:

Who?s Using Ajax
"Google is making a huge investment in developing the Ajax approach.
All of the major products Google has introduced over the last year ?
Orkut, Gmail, the latest beta version of Google Groups, Google
Suggest, and Google Maps ? are Ajax applications. Others are following
suit: many of the features that people love in Flickr depend on Ajax,
and Amazon?s A9.com search engine applies similar techniques."

Ajax: A New Approach to Web Applications - by Jesse James Garrett 
Adaptive Path
http://www.adaptivepath.com/publications/essays/archives/000385.php

Rainbow~

You might be interested in this official site.

http://code.google.com/

Best Regards

Google hire many ppl, but for all position they ask for knowledge
python and C. Think so, they write systems with that languages.