Should I detect language from browser or OS?

I’m creating a multilingual website and need to display translated text to the user. There are many ways to detect language with Javascript, but I don’t know which method is the best. I’ve never used a non-English OS or browser, so I don’t know how foreign people set up their computers.

[LIST]
[]navigator.language - all non-IE browsers
[
]navigator.browserLanguage IE-only - Browser Localized Language
[]navigator.systemLanguage IE-only - OS Localized Language
[
]navigator.userLanguage IE-only - OS Regional Settings
[/LIST]

For non-IE browsers, we only have one choice, so it’s an easy decision. For IE browsers, we have three choices. Which one should we use? What is the likelihood that the three choices are out of synch? When you install a browser, does it automatically set its language to be the same as the OS language?