gistfile1.js

var express = require('express');
var i18n = require('i18n-2');
var app = express();
 
var localeSetup = {
    locales: ['en', 'es'],
    defaultLocale: 'en'
}
 
var setNewLocaleFromQuery = function(req, res, next) {
    if (req.query.lang) {
        req.i18n.setLocaleFromQuery(req);
    }
    next();
}
 
module.exports = exports = function(app) {
	i18n.expressBind(app, localeSetup);
	app.use(setNewLocaleFromQuery);
};

// and in your main app:
require('thisfile')(app);