While I’d try to do that in mod_rewrite code (in .htaccess), I can’t think of how to access the browser’s default language off the top of my head. On the other hand, there are major websites which use mod_negotiation for exactly what you’re trying to do. Unfortunately, I have not studied mod_negotiation so I have to simply recommend that you go to apache.org and look at:
[indent]Content-Language:
The language(s) of the variant, as an Internet standard language tag (RFC 1766). An example is en, meaning English. If the variant contains more than one language, they are separated by a comma.[/indent]