router.js

Router.configure({
  layoutTemplate: 'layoutBackend',
  notFoundTemplate: 'page404',
  loadingTemplate: 'loading',
  i18n: {
    languages: TP.Const.AllowedLangKey,

    setLanguage: function (lang) {
      if (Meteor.isClient) {
        TP.Core.Functions.setLanguageKey(lang);
      }
    },
    missingLangCodeAction: function (url) {
      if (Meteor.isClient) {

        TP.initLangCodeMissing = true;

        // pasted code from origin i18n
        var lang = this.getLanguage();

        var path = Iron.Url.normalize(url);

        if (lang) {
          this.go('/' + lang + path);
          return true;
        } else {
          console.log("Can't retrieve current language on " + this.getEnv() + ".");
          return false;
        }
      }
    }
  }
});

template.html

Template.body.rendered = function () {
  this.autorun(function () {
    var langCodeMissing = TP.initLangCodeMissing;
    var userLang = Meteor.user() && Meteor.user().languageKey;
    var currentRouterLanguage = Router.getLanguage();
    if (langCodeMissing && !Meteor.loggingIn() && userLang) {
      if (userLang !== currentRouterLanguage) {
        Router.setLanguage(userLang);
      }
    } else {
      if (currentRouterLanguage !== TP.Core.Functions.getLanguageKey()) {
        TP.Core.Functions.setLanguageKey(currentRouterLanguage);
      }
    }
  });
};