server.js

const express = require ('express');
const path = require ('path');
const next = require ('next');

const dev = process.env.NODE_ENV !== 'production';
const app = next ({dev});
const handle = app.getRequestHandler ();

const i18nextMiddleware = require ('i18next-express-middleware');
const Backend = require ('i18next-node-fs-backend');
const {i18nInstance} = require ('./i18n');

// init i18next with serverside settings
// using i18next-express-middleware
i18nInstance.use (Backend).use (i18nextMiddleware.LanguageDetector).init ({
  fallbackLng: 'en',
  preload: ['en'], // preload all langages
  ns: ['common', 'home', 'about' , 'landing-page', 'login', 'register', 'fuel-wise', 'public-transport', 'addpost', 'editpost', 'showpost', 'registration-succesful', 'sign-in'/* new-i18n-namespace-here */],
  backend: {
    loadPath: path.join (__dirname, '/static/locales/{{lng}}/{{ns}}.json'),
    addPath: path.join (
      __dirname,
      '/static/locales/{{lng}}/{{ns}}.missing.json'
    ),
  },
  debug: false,
}, () => {
  // loaded translations we can bootstrap our routes
  app.prepare ().then (() => {
    const server = express ();

    // enable middleware for i18next
    server.use (i18nextMiddleware.handle (i18nInstance));

    // serve locales for client
    server.use (
      '/locales',
      express.static (path.join (__dirname, '/static/locales'))
    );

    // missing keys
    server.post (
      '/locales/add/:lng/:ns',
      i18nextMiddleware.missingKeyHandler (i18nInstance)
    );

    // use next.js
    server.get ('*', (req, res) => handle (req, res));

    server.listen (3000, err => {
      if (err) throw err;
      console.log ('> Ready on http://localhost:3000');
    });
  });
});