All browsers support it. We should document that somewhere and somewhere might as well be where the Navigator object is defined.
Maybe "en" should be used as value when the user wants to reduce fingerprinting. Though given all the other exposed fingerprinting data this might not be worth mentioning.
$ grep -aPhc "navigator\.language" web200904
$ grep -aPhc "navigator\.language" stevef-all
I quickly looked through the stevef-all (which is from http://www.paciellogroup.com/blog/2012/04/html5-accessibility-chops-data-for-the-masses/ ) matches. Most or all of them also use navigator.userLanguage or navigator.browserLanguage or navigator.systemLanguage if navigator.language is not defined. The common usage of this API seems to be for tracking, although there are probably some that use it for localization purposes as well.
This bug was cloned to create bug 17878 as part of operation convergence.
Mass move to "HTML WG"
EDITOR'S RESPONSE: This is an Editor's Response to your comment. If you are
satisfied with this response, please change the state of this bug to CLOSED. If
you have additional information and would like the Editor to reconsider, please
reopen this bug. If you would like to escalate the issue to the full HTML
Working Group, please add the TrackerRequest keyword to this bug, and suggest
title and text for the Tracker Issue; or you may create a Tracker Issue
yourself, if you are able to do so. For more details, see this document:
Change Description: No spec change.
Rationale: This has already been added to the spec, as you can see here: