How can I get the browser language in node.js (express.js)?

User requests some page and I want to know (on server side) what is the language in his/her browser. So I could render template with the right messages.

On client side it's easy:

var language = window.navigator.userLanguage || window.navigator.language

You can use req.headers["accept-language"] to get the language/locale the user has set in his browser.

For easier support, you may want to look into a https://github.com/jed/locale">locale module.

request.acceptsLanguages will contain a parsed version of request.headers['accept-language'].

See: http://expressjs.com/en/api.html#req.acceptsLanguages

With Express 4.x, you can use the build in req.acceptsLanguages(lang [, ...]) to check if certain languages are accepted.

var express = require('express');
app.get('/translation', function(request, response) {
    var lang = request.acceptsLanguages('fr', 'es', 'en');
    if (lang) {
        console.log('The first accepted of [fr, es, en] is: ' + lang);
        ...
    } else {
        console.log('None of [fr, es, en] is accepted');
        ...
    }
});

To get the list of all accepted languages, using Express 4.x, you can use the module accepts.

var express = require('express'), accepts = require('accepts');
app.get('/translation', function(request, response) {
    console.log(accepts(request).languages());
    ...
});

Middleware to set the request language and use it globally:

// place this middleware before declaring any routes
app.use((req, res, next) => {
	// This reads the accept-language header
	// and returns the language if found or false if not
	const lang = req.acceptsLanguages('bg', 'en')
    
    if (lang) { // if found, attach it as property to the request
        req.lang = lang
    } else { // else set the default language
        req.lang = 'en'
    }

    next()
})

Now you can access 'req.lang'

app.get('/', (req, res) => {
	res.send(`The request language is '${req.lang}'`)
})

Example using translation

const translate = {
	en: {
		helloWorld: "Hello World!"
	},
	bg: {
		helloWorld: "Здравей Свят!"
	}
}
app.get('/hello-world', (req, res) => {
	res.send(translate[req.lang].helloWorld)
})

You would need to parse the string in req.headers["accept-language"]. Which will give you a priority list of preferred languages from the client. You can also check req.acceptsLanguages(lang [, ...]) if your language is supported or not.

I would strongly recommend to use [express-request-language][1] to do any language matching work, since it could be very difficult to get it right at the first time.

Most of the time, matching a language is not enough. A user might want to change a preferred language. express-request-language help you store a preferred language in a cookie it also gives your server a URL path to change a preferred language.

All above functionality can be done with just a couple of lines of code:

app.use(requestLanguage({
  languages: ['en-US', 'zh-CN'],
  cookie: {
    name: 'language',
    options: { maxAge: 24*3600*1000 },
    url: '/languages/{language}'
  }
}));

In case of no match, the middleware will also match a default language (en-US above). [1]: https://github.com/tinganho/express-request-language