The question is how many people set (or have by default) right language
in browsers ?
The download page of Opera, Firefox or IE provide you with the localized
version of the program by default, at least that works from Argentina.
So I assume most people have it but they don't know that it is there anyway.
I've learned to have all my programs in English and that part of content
negotiation is a pain for me.
Also, the flash player plug-in download page offers a localized version
of the binary based on the IP geographic location).