server.js

import requestLanguage from 'express-request-language';
import models, { Locale, User, UrlManager } from './data/models';

app.use(async (res, req, next) => {

  const locales = await Locale.findAll({}).then(locales => locales.map(loc => loc.locale));
  //app.set('locales', locales);
  req.locales = locales;
  
  next();

});

app.use((req, res, next) => {
  return requestLanguage({
    languages: req.locales, // unable to get array
    queryName: 'lang',
    cookie: {
      name: 'lang',
      options: {
        path: '/',
        maxAge: 3650 * 24 * 3600 * 1000, // 10 years in miliseconds
      },
      url: '/lang/{language}',
    },
  })(req, res, next);
});