There are various problems in your code.

• querySelector returns one element, and not an array, so document.querySelector("head")[0]makes no sense.
• With if (head.classList.contains("En-Script") == true) { you test if head has the class En-Script, which makes no sense in combination with head.classList.add("En-Script")
• Also adding a class to the head to check if the script is loaded is probably not the best approach, if you want to do that add a class or data attribute to the script element you append.
• If you already have head why do you use let EnScript = document.querySelector(".En-Script"); to query for it again?
• EnScript.appendChild("script"); tries to append test string with the content script to EnScript and not the element you created and stored in the script variable.
• $(document).ready(function() { would not wait for the script to be loaded.
• Calling .ready( for when you loaded jQuery dynamically also does not make much sense because the DOM is already read.

That's how loading a script could look like:

(function() {
  var head = document.querySelector("head");

  var script = document.createElement("script");
  script.type = "text/javascript";

  script.onload = function() {
    // called when the script is loaded and ready to use
    console.log("jquery added successfully")
  }
  script.src = "https://code.jquery.com/jquery-3.6.0.slim.min.js";

  head.appendChild(script);

})();

There's a few things wrong, you were checking if head had the class "En-Script" and then adding the class to the head, so I changed the validation so it makes sense. Then, you were selecting the head using querySelector which only returns 1 value and using [0] which would work if you were using querySelectorAll instead.

Then you should add the jquery function $(document) after the script has loaded, so I added a script.onload to achieve this.

Also when you append a child you should send the object you created, not a string, so instead of this EnScript.appendChild("script"); I used head.appendChild(script);

(function() {

  var head = document.querySelector("head");
  if (!head.classList.contains("En-Script")) {
    head.classList.add("En-Script");
    var script = document.createElement('script');
    script.type = "text/javascript";
    script.src = "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js";
    script.onload = function() {
      $(document).ready(function() {
        console.log("jquery added successfully")
      });

    };
    head.appendChild(script);
  } else {
    console.log("class_list_added_for_further_process");
  }


})();