index.js

var geoip = require('geoip-ultralight');
var countries = require('country-data').countries;
var languages = require('country-data').languages;

function getCountryCode(ip, defaultCountry){
		
	if (ip.indexOf('::ffff:') > -1) {
		ip = ip.split(':').reverse()[0];
	}
	var countryCode;
	var lookedUpCountry = geoip.lookupCountry(ip);
	if (lookedUpCountry) {
		countryCode = lookedUpCountry;
	}
	if ((ip === '127.0.0.1') || (!lookedUpCountry)) {
		countryCode = defaultCountry;
	}
	return countryCode;
}

exports = module.exports = function (opts) {

	var cookieLangName = opts.cookieLangName || "ulang";
	var defaultCountry = opts.defaultCountry || "US";
	var siteLangs = opts.siteLangs || ['en'];

	if (siteLangs.constructor !== Array) throw new Error('siteLangs must be an Array with supported langs.');

	return function geolang(req, res, next) {

		// Geo data isn't in locals
		if(!('countryLangData' in req.app.locals)) {

			var ip = req.headers['x-forwarded-for'] || req.connection.remoteAddress;
			var countryCode=getCountryCode(ip, defaultCountry);
			var countryData=countries[countryCode];
			var countryLang = languages[countryData.languages[0]];
			
			req.app.locals.countryData=countryData;
			req.app.locals.countryLangData=countryLang;
			req.app.locals.countryLang=countryLang.alpha2;
		}

		// Lang isn't already set so skip
		if (!(cookieLangName && req.session && req.session[cookieLangName])) {
			if (siteLangs.indexOf(req.app.locals.countryLang) > -1) {
				req.session[cookieLangName]=req.app.locals.countryLang;
			}
		}

		next();
    };
};