How to determine user's locale within browser?

Question

The proper way is to look at the HTTP Accept-Language header sent to the server. This contains the ordered, weighted list of languages the user has configured their browser to prefer.

Unfortunately this header is not available for reading inside JavaScript; all you get is navigator.language, which tells you what localised version of the web browser was installed. This is not necessarily the same thing as the user's preferred language(s). On IE you instead get systemLanguage (OS installed language), browserLanguage (same as language) and userLanguage (user configured OS region), which are all similarly unhelpful.

If I had to choose between those properties, I'd sniff for userLanguage first, falling back to language and only after that (if those didn't match any available language) looking at browserLanguage and finally systemLanguage.

If you can put a server-side script somewhere else on the net that simply reads the Accept-Language header and spits it back out as a JavaScript file with the header value in the string, eg.:

var acceptLanguage= 'en-gb,en;q=0.7,de;q=0.3';

then you could include a <script src> pointing at that external service in the HTML, and use JavaScript to parse the language header. I don't know of any existing library code to do this, though, since Accept-Language parsing is almost always done on the server side.

Whatever you end up doing, you certainly need a user override because it will always guess wrong for some people. Often it's easiest to put the language setting in the URL (eg. http​://www.example.com/en/site vs http​://www.example.com/de/site), and let the user click links between the two. Sometimes you do want a single URL for both language versions, in which case you have to store the setting in cookies, but this may confuse user agents with no support for cookies and search engines.

On Chrome and Firefox 32+, navigator.languages contains an array of locales in order of user preference, and is more accurate than navigator.language, however, to make it backward-compatible (Tested Chrome / IE / Firefox / Safari), then use this:

function getLang() {
  if (navigator.languages != undefined) 
    return navigator.languages[0]; 
  return navigator.language;
}

This article suggests the following properties of the browser's navigator object:

• navigator.language (Netscape - Browser Localization)
• navigator.browserLanguage (IE-Specific - Browser Localized Language)
• navigator.systemLanguage (IE-Specific - Windows OS - Localized Language)
• navigator.userLanguage

Roll these into a javascript function and you should be able to guess the right language, in most circumstances. Be sure to degrade gracefully, so have a div containing your language choice links, so that if there is no javascript or the method doesn't work, the user can still decide. If it does work, just hide the div.

The only problem with doing this on the client side is that either you serve up all the languages to the client, or you have to wait until the script has run and detected the language before requesting the right version. Perhaps serving up the most popular language version as a default would irritate the fewest people.

Edit: I'd second Ivan's cookie suggestion, but make sure the user can always change the language later; not everyone prefers the language their browser defaults to.

Combining the multiple ways browsers are using to store the user's language, you get this function :

const getNavigatorLanguage = () => {
  if (navigator.languages && navigator.languages.length) {
    return navigator.languages[0];
  } else {
    return navigator.userLanguage || navigator.language || navigator.browserLanguage || 'en';
  }
}

We first check the navigator.languages array for its first element.
Then we get either navigator.userLanguage or navigator.language.
If this fails we get navigator.browserLanguage.
Finally, we set it to 'en' if everything else failed.

---

And here's the sexy one-liner :

const getNavigatorLanguage = () => (navigator.languages && navigator.languages.length) ? navigator.languages[0] : navigator.userLanguage || navigator.language || navigator.browserLanguage || 'en';

There's a difference between the user's preferred languages and the system/browser locale.

A user can configure preferred languages in the browser, and these will be used for navigator.language(s), and used when requesting resources from a server, to request content according to a list of language priorities.

However, the browser locale will decide how to render number, date, time and currency. This setting is likely the highest ranking language, but there is no guarantee. On Mac and Linux, the locale is decided by the system regardless of the user language preferences. On Windows is can be elected among the languages in the preferred list on Chrome.

By using Intl (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl), developers can override/set the locale to use to render these things, but there are elements that cannot be overridden, such as the <input type="date"> format.

To properly extract this language, the only way I've found is:

(new Intl.NumberFormat()).resolvedOptions().locale

(Intl.NumberFormat().resolvedOptions().locale also seems to work)

This will create a new NumberFormat instance for the default locale and then reading back the locale of those resolved options.

I used all the answers and created a single line solution:

const getLanguage = () => navigator.userLanguage || (navigator.languages && navigator.languages.length && navigator.languages[0]) || navigator.language || navigator.browserLanguage || navigator.systemLanguage || 'en';

console.log(getLanguage());

I did a bit of research regarding this & I have summarised my findings so far in below table

So the recommended solution is to write a a server side script to parse the Accept-Language header & pass it to client for setting the language of the website. It's weird that why the server would be needed to detect the language preference of client but that's how it is as of now There are other various hacks available to detect the language but reading the Accept-Language header is the recommended solution as per my understanding.