[soneil]

I was playing with this today: at least on OSX, I couldn't find a browser that reasonably handled "I speak more than one language comfortably" out of the box (eg, using the OS settings).

Safari: sends the primary OS/environment language, and nothing else. (the OS allows for a weighted list of multiple languages, Safari honours only the primary language). There is no configuration option for this.

Chrome: sends the primary OS/environment language, then en_US, then en. If you had a second preference that wasn't english (eg Belgium, Switzerland, etc), tough. It is atleast configurable (if you look hard enough)

Firefox: entirely ignores the OS environment; uses the localization you downloaded. But is also configurable.

[Manishearth]

What I love about Firefox in this space is that it lets you configure fonts per-language, which is important because default fonts are often not that great.

I've never found sites that actually use Accept-Language properly, so I care as much if that works well.