submit（）重复执行，php只接收默认值

I have a submit form using POST method, without a submit button.

<form action="" method="post" id="submit_cookie_form">
   <input type="hidden" name="lang" value="" id="cookie_form">
</form>

My goal:

1. Pass value to input hidden value attribute[*1]
2. Set the code to be executed/triggered when a value is passed to value attribute[*2]
3. Submit the form when a value is passed to value attribute[*3]=>ISSUE
4. Pass Javascript value('en' or 'jp') to php variable. [*4]=>ISSUE

[*3] => When I use submit() as follows to [*3], the page is being reloaded forever. It seems like the form is being submitted forever. Why does this happen, and how can I fix the code?

[*4] => Why does php only receive 'en' value? Although, value attribute of <input type="hidden" name="lang" value="" id="cookie_form"> perfectly recieves 'en' and 'jp' according to user's clicks on the webpage. How can I fix this?

 let latestCookie;

    $(document).on('ready', function(){

       if (localStorage.getItem("language") === null || localStorage.getItem("language") === flag.en){
         document.cookie = "language=en";

       } else if (localStorage.getItem("language") === flag.jp){
         document.cookie = "language=jp";
       }

        latestCookie = getCookieValue('language');
        document.getElementById('cookie_form').value = latestCookie; //[*1]
        setLang(latestCookie); //[*2]
     })


      $('#cookie_form').change(function(){
          $('#submit_cookie_form').submit(); }); //[*3]

      function setLang(chosenLang){
          $('#cookie_form').val(chosenLang).trigger('change');
        }

   <?php 
         if(isset($_POST['lang'])){     
           $lang=$_POST['lang'];   //[*4]
           echo $lang;
         }
         else{
           echo $lang;
         }
   ?>

$('.dropdown-menu a img').on('click', function(){ 
    let flag = $(this).attr('src'); 
    $('#selected').attr('src', flag); 
    let key = $('#selected').attr('src'); 
    localStorage.setItem("language", key); 
    refreshPage(key); 
});
function refreshPage(key){
    window.location.href = '<php echo $_SERVER[\'PHP_SELF\']."?lang="; ?>'+key;
}

then in PHP you can get in GET method as a querystring.

<?php 
     if(isset($_REQUEST['lang'])){     
       $lang=$_REQUEST['lang'];   //[*4]
       echo $lang;
     }
     else{
       echo $lang;
     }
?>