The problem of user language lists in JavaScript

Edit: I wrote a followup post to this one at [Follow-up on navigator.languages]

I've always wondered why it is not possible to get in JavaScript the list of all languages as configured in the browser. This list is made available to servers via a HTTP header.

On the other hand, JavaScript can only get the first language using:

console.log(navigator.language); // 'en', 'fr', 'de' or whatever

Browsers inconsistencies

Getting the language of the browser is part of the HTML5 specifications, but implementations vary widely.

Internet Explorer

Let's start with IE that has (surprisingly) the most complete set of (non standard) features about the environment language. navigator.userLanguage gets the first language set by the user (can be changed in Internet Options > General > Languages ; see Internet Explorer Dev Center).

navigator.browserLanguage returns the language of the UI of the browser. You can't change this value, it is decided by the version of the executable you installed (This property and all its subtleties are described in Internet Explorer Dev Center).

Finally, navigator.systemLanguage will give you the locale used by the OS (See Internet Explorer Dev Center.

Firefox & Safari

navigator.language returns the first language in the list of languages.

In Firefox, you can define it in Options > Content > Languages.

Safari uses the language set at the system level (See navigator.systemLanguage of IE above). You cannot just override it in Safari.

Some details are available on the navigator.language page on MDN.

Chrome

Invariantly returns the language of the UI through navigator.language without a possibility to change it. The value is similar to navigator.browserLanguage of IE.

Chrome extensions can retrieve the full list of languages as set by the user thanks to chrome.i18n.getAcceptLanguages() (Not sure why this API is async):

chrome.i18n.getAcceptLanguages(function(requestedLocales) {
  // 'requestedLocales' is an array of strings.
});

What's wrong with the current approach?

Well, apart from the semantic inconsistencies, knowing only the main language is a serious limitation. Let's illustrate this with my personal experience. My browsers have the following configuration:

  1. Japanese
  2. English (GB)
  3. English
  4. English (US)
  5. French
  6. Spanish
  7. Korean
  8. German

(Yeah, I want to train my Japanese, so I listed it first!)

A website available in several languages will show the interface in Japanese and English if not available.

But a local app will only get the first language, Japanese. So if this language is not available, the UI will be shown in a totally random language that I may not understand.

Also, if the app knows the full list, it could use it to detect visitors speaking rare languages and ask them to collaborate on translation.

How to fix it?

First of all, an easy cross browser way to get the preferred language is to do:

/** @const */ DEFAULT_VALUE = 'en';
/** @const */ PREFERRED_LANGUAGE = navigator.language || navigator.userLanguage || navigator.browserLanguage || navigator.systemLanguage || DEFAULT_VALUE;

That's fine as long as your app supports this language.

If not, you could always ping a server, get the accept-language HTTP header, send the response back to JavaScript and parse it. That means that you need a scriptable server, so you cannot do it for, offline apps or for apps hosted on a static web server like Github Pages. Oh and Mac have only one language configured at a time: you will never get more than 1 element in the list of this header on Safari.

What to do next?

The good news is things are being worked on.

First, there is this bug on the WHATWG bug tracker to discuss this feature, with all the details here. Firefox is already thinking of implementing it.

To put it in a nutshell, the current consensus is to have a new JavaScript property called navigator.languages that returns an array of language codes sorted by preference order:

console.log(navigator.languages);
// The configuration above would give something like:
// ['ja', 'en-GB', 'en', 'en-US', 'fr', 'es', 'ko', 'de'];

Also, firing an event when language changes is also being discussed. Firefox OS allows this on certified apps with a proprietary API:

navigator.mozSettings.addObserver('language.current', function(event) {
  console.log('New language', event.settingValue); // Default value is 'en-US'.
});

To conclude

Making the full list of languages configured in a browser available to JavaScript can't be done front-end at all and partially using back-end. But this information is stored in the OS/browser itself. Am I the only one to think something is wrong here?

This topic is hard and has been around for a while, so some of the resources I consulted might be outdated. Use the comments below if you spotted any error.