How can a site still determine my browser language?

[Steven]

Hi,

I have been using Privoxy and Tor for a while and came across this  
strange phenomenon lately. Privoxy is configured to
- hide-accept-language
   Fake these language settings: en_US
- hide-referrer
   Fake as the root directory of the site
- hide-user-agent
   User Agent string to send:
   Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
- session-cookies-only
   (and I delete cookies manually anyway before each try)

I am located in the US and my real browser is Firefox 1.5 German  
version for Mac OS X. I use an US-based exit node on Tor, so geo- 
locating the IP should come back with a US location. Javascript is  
entirely turned off.

Yet, each time I go to www.gmail.com, Google redirects me to the  
German version of Gmail. I think the only reason this happens is the  
fact that my real browser is German. But how can Google detect that?  
There are no other relevant URL patterns that match for Google on  
Privoxy (i.e. no exception rules) as far as I can tell. Any ideas?

Thanks,
Steven