Переключить язык сайта

.lang { 
  display: none; 
} 
 
.lang.visible { 
  display: inline; 
  visibility: hidden; 
  font: 1em sans-serif; 
}

 <div id="languages"> 
    <label> 
      <input type="radio" name="lang">RU 
    </label> 
    <label> 
      <input type="radio" name="lang">ENG 
    </label> 
  </div> 
 
  <div id="welcome-message"> 
    <span class="lang lang-ru">Привет</span> 
    <span class="lang lang-eng">Hello</span> 
    , username! 
 
    <span class="lang lang-ru">Рады тебя видеть!</span> 
  </div> 
 
  <button id="save">Save</button>

var applyLanguage = function (lang) { 
	alert('now language is: ' + lang); 
} 
 
var getCurrentLanguage = function () { 
	var defaultLanguage = 'ru'; 
	// 
   
  return defaultLanguage; 
} 
 
var currentLang = getCurrentLanguage(); 
var langEls = document.getElementsByClassName('lang-' + currentLang); 
 
for (var i=0; i<langEls.length; i++) { 
	var langEl = langEls[i]; 
	 
  // langEl.style.display = 'inline'; 
  langEl.classList.add('visible'); 
} 
 
 
 
 
 
// $<prefix> == DOMElement 
var $save = document.querySelector('html body button#save') 
 
$save.addEventListener('click', function(){ 
	alert(1); 
});