You were missing a protocol. Also, here's a suggestion for a cleaner approach replacing the series of if's.

var userLang = navigator.language || navigator.userLanguage;
var urls = {
    'fr': 'http://www.facebook.com',
    'de': 'http://www.facebook.com',
};
var defaultUrl = 'http://www.google.com';

var url = urls[userLang] ?? defaultUrl;
console.log(url);

//window.location.href = url;

// Better code in terms of lines of code and avoiding if conditions.

const sites = {
'fr': 'https://www.facebook.com',
'de': 'https://www.google.com',
};

const userLang = navigator.language || navigator.userLanguage;
window.location.href = sites[userLang];

Please give it a try like this:

<script>
  $( document ).ready(function(){
    var userLang = navigator.language || navigator.userLanguage;
    if (userLang == "fr") {
        window.location.href = "https://www.facebook.com"
    }
    else if(userLang == "de"){
       window.location.href = "https://www.facebook.com"
    }
    else {
        window.location.href = "https://www.google.com"
    }
});
</script>

And find this documentation MDN HTML Script tag regarding how to use the tag.