index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
  </head>
  <body>
    <script>
      //Language given in translation
      const langs = ['fr', 'en'];
      const defLang = 'en';

      //Get the favorite language of the user's navigator
      const userLangs = (navigator.languages || [navigator.language]).map(
        a => a.split('-').shift(),
      );
      userLangs.push(defLang);
      const selectedLang = userLangs.find(lang => langs.indexOf(lang) !== -1);

      //Redirection to the appropriate index.html
      document.location.replace(selectedLang);
    </script>
  </body>
</html>