JavaScript for detecting browser language preference

I think the main problem here is that the browser settings don't actually affect the navigator.language property that is obtained via javascript.

What they do affect is the HTTP 'Accept-Language' header, but it appears this value is not available through javascript at all. (Probably why @anddoutoi states he can't find a reference for it that doesn't involve server side.)

I have coded a workaround: I've knocked up a google app engine script at http://ajaxhttpheaders.appspot.com that will return you the HTTP request headers via JSONP.

(Note: this is a hack only to be used if you do not have a back end available that can do this for you. In general you should not be making calls to third party hosted javascript files in your pages unless you have a very high level of trust in the host.)

I intend to leave it there in perpetuity so feel free to use it in your code.

Here's some example code (in jQuery) for how you might use it

$.ajax({ 
    url: "http://ajaxhttpheaders.appspot.com", 
    dataType: 'jsonp', 
    success: function(headers) {
        language = headers['Accept-Language'];
        nowDoSomethingWithIt(language);
    }
});

Hope someone finds this useful.

Edit: I have written a small jQuery plugin on github that wraps this functionality: https://github.com/dansingerman/jQuery-Browser-Language

Edit 2: As requested here is the code that is running on AppEngine (super trivial really):

class MainPage(webapp.RequestHandler):
    def get(self):
        headers = self.request.headers
        callback = self.request.get('callback')

        if callback:
          self.response.headers['Content-Type'] = 'application/javascript'
          self.response.out.write(callback + "(")
          self.response.out.write(headers)
          self.response.out.write(")")
        else:
          self.response.headers['Content-Type'] = 'text/plain'
          self.response.out.write("I need a callback=")

application = webapp.WSGIApplication(
                                     [('/', MainPage)],
                                     debug=False)

def main():
    run_wsgi_app(application)

if __name__ == "__main__":
    main()

Edit3: Have open sourced the app engine code here: https://github.com/dansingerman/app-engine-headers

Update of year 2014.

Now there is a way to get Accept-Languages in Firefox and Chrome using navigator.languages (works in Chrome >= 32 and Firefox >= 32)

Also, navigator.language in Firefox these years reflects most preferred language of content, not language of UI. But since this notion is yet to be supported by other browsers, it is not very useful.

So, to get most preferred content language when possible, and use UI language as fallback:

navigator.languages
    ? navigator.languages[0]
    : (navigator.language || navigator.userLanguage)

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.

I came across this piece of code to detect browser's language in Angular Translate module, which you can find the source here. I slightly modified the code by replacing angular.isArray with Array.isArray to make it independent of Angular library.

var getFirstBrowserLanguage = function () {
    var nav = window.navigator,
    browserLanguagePropertyKeys = ['language', 'browserLanguage', 'systemLanguage', 'userLanguage'],
    i,
    language;

    // support for HTML 5.1 "navigator.languages"
    if (Array.isArray(nav.languages)) {
      for (i = 0; i < nav.languages.length; i++) {
        language = nav.languages[i];
        if (language && language.length) {
          return language;
        }
      }
    }

    // support for other well known properties in browsers
    for (i = 0; i < browserLanguagePropertyKeys.length; i++) {
      language = nav[browserLanguagePropertyKeys[i]];
      if (language && language.length) {
        return language;
      }
    }

    return null;
  };

console.log(getFirstBrowserLanguage());