<h2 class="title"><span>Меню</span> сайта</h2>

$('h2.title').each(function() {    
var text= $(this).text()
var index = h.indexOf(' ');    
      if(index == -1) {    
               index = h.length;    
           }    
$(this).html('<h2 class="title"><!-- <bt> --><span>' + text.substring(0, index) + '</span>' + text.substring(index, h.length)+'<!-- </bt> --></h2>');    
       });    
    

<h2 class="title"><!-- <bt> -->Меню сайта<!-- </bt> --></h2>

<!-- <bt> -->

<script>
+function(){
  var elems = document.getElementsByTagName('h2'),
   count = elems.length;
  while(count--){
   if(/(\b|^)title(\b|$)/.test(elems[count].className)){
    elems[count].innerHTML=elems[count].innerHTML.replace(/-->([^\s]+?)\s/,'--><span>$1</span> ');
    }
   }
  }()
</script>