Определение языка браузера и отображение нужной страницы

Смотрите в объекте navigator свойства language или userLanguage.

var lang = navigator.language || navigator.userLanguage; 

    let language = window.navigator.language;
let languageFistTwo = language.substr(0,2); // To only keep the first 2 characters.
let currentLocation = document.getElementsByTagName('html')[0].getAttribute('lang-js')

switch (languageFistTwo) { 
  case "en":
   if(currentLocation != 'en'){
      window.location.href = 'index_en.html';
   }
    break;

  case "sv":
     if(currentLocation != 'sv')
   window.location.href = 'index_sa.html';
    break;

  case "tr":
     if(currentLocation != 'tr')
   window.location.href = 'index_tr.html';
    break;

  case "uk":
     if(currentLocation != 'uk')
   window.location.href = 'index_ua.html';
    break;

  default:
     if(currentLocation != 'ru'){
      window.location.href = 'index.html';
     }
}