if (navigator.browserLanguage) { // Internet-Explorer
  if (navigator.browserLanguage.indexOf("de") > -1) {
    window.location.href="de/s1.htm";
  } else {
    window.location.href="en/s1.htm";
  };
} else { // andere Browser
  if (navigator.language.indexOf("de") > -1) {
    window.location.href="de/s1.htm";
  } else {
    window.location.href="en/s1.htm";
  };
};