JavaScript for detecting browser language preference [duplicate]

I have been trying to detect the browser language preference using JavaScript.

If I set the browser language in IE in Tools>Internet Options>General>Languages, how do I read this value using JavaScript?

Same problem for Firefox. I’m not able to detect the setting for tools>options>content>languages using navigator.language.

Using navigator.userLanguage , it detects the setting done thru
Start>ControlPanel>RegionalandLanguageOptions>Regional Options tab.

I have tested with navigator.browserLanguage and navigator.systemLanguage but neither returns the value for the first setting(Tools>InternetOptions>General>Languages)

I found a link which discusses this in detail, but the question remains unanswered 🙁

var language = window.navigator.userLanguage || window.navigator.language;
alert(language); //works IE/SAFARI/CHROME/FF

window.navigator.userLanguage is IE only and it’s the language set in Windows Control Panel – Regional Options and NOT browser language, but you could suppose that a user using a machine with Window Regional settings set to France is probably a French user.

navigator.language is FireFox and all other browser.

Some language code: 'it' = italy, 'en-US' = english US, etc.

As pointed out by rcoup and The WebMacheter in comments below, this workaround won’t let you discriminate among English dialects when users are viewing website in browsers other than IE.

window.navigator.language (Chrome/FF/Safari) returns always browser language and not browser’s preferred language, but: “it’s pretty common for English speakers (gb, au, nz, etc) to have an en-us version of Firefox/Chrome/Safari.” Hence window.navigator.language will still return en-US even if the user preferred language is en-GB.