A simple example of using ES6 tagged template strings for localization purposes.

localization.js

var language = 'ru';

//--------------
const langDatabases = {
  en: null,
  ru: new Map(),
  pl: new Map()
}

function strKey(tplArr){
  return tplArr.raw.join('$${}');
}

function tran(from, ...toRest){
  if(
    from.lang !==  'en' ||
    toRest.some(to => from.strings.length !== to.strings.length)
  ){
    return false;
  }
  let fromKey = strKey(from.strings);
  toRest.forEach((to) => {
    langDatabases[to.lang].set(fromKey, to.strings);
  });
  return true;
}

const en = strings => ({lang: 'en', strings});
const ru = strings => ({lang: 'ru', strings});
const pl = strings => ({lang: 'pl', strings});

tran(
  en`Hello, ${'userName'}! Good to see you.`,
  ru`Привет, ${'userName'}! Рады видеть вас.`,
  pl`Witam, ${'username'}! Cieszę się, że cię widzę.`,
);

//----------------------
function loc(strings, ...vars){
  if(language !== 'en'){
    strings = langDatabases[language].get(strKey(strings));
  }

  return strings.reduce((accumulator, part, i) => {
    return accumulator + vars[i - 1] + part
  });
}

//----------------------
var userName = 'Ruslan';
console.log('Message (in local)', loc`Hello, ${userName}! Good to see you.`);