Forum Moderators: open
I see what you mean and that it would be good service for your users if they automatically get redirected to the appropriate language. However my own feel is that I what to choose what language I want to see.
Secondly you could run into problems with the search engines, if you run an automatic redirect. I would use flags in stead - give the users an option to click on the flag/language of their choice.
The javascript object to read is not standardized, as far as I know -- it's either navigator.language in Netscape or navigator.userLanguage in Explorer. Not sure about Opera / Mozilla etc but a bit of experimenting should tell you which is which. I doubt that any browser invented yet another object name.
IE, because of its infamous integration with the operating system, can also read the system language through the systemLanguage object. Here's a Microsoft reference [msdn.microsoft.com] on the topic.
Because so many computers are shipped with English as the default language, and so many users do not change this default, you cannot count on the user's preference to be indicated by the brower when it says English. So I agree with Rumbas, keep the national flags. And don't re-direct every visitor because that certainly can get you in trouble with the search engines.
I'd suggest having one default page in your most common visitor languge. Then from that page, use an external javascript to sniff for the browser langauge and re-direct those who are clearly different.