Пустой $_POST после AJAX запроса

$('#form li input').click(function select_lang() { 
  var lang = $(this).attr('id'); 
  $.ajax({ 
    type: "POST", 
    url: "php/header.php", 
    data: { 
      'lang': lang 
    }, 
    cache: false, 
    success: function(response) { 
      console.log(response); 
    }, 
    error: function(error) { 
      console.log(error); 
    } 
  }); 
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> 
<ul> 
  <form method="post" action="javascript:void(null);" id="form" onsubmit="select_lang()"> 
    <li> 
      <input type="button" name="lang" id="first" value="Hello"> 
    </li> 
    <li> 
      <input type="button" name="lang" id="second" value="Bye"> 
    </li> 
  </form> 
</ul>