<script language="javascript">
function changeDIR(id)
{
   var input   = document.getElementById(id);
   var current = input.attributes['dir'].value;

   if (current == "")
      input.setAttribute('dir', 'rtl');

   if (current == "rtl") {
        input.attributes['dir'].value = "ltr";
        document.getElementById('change').value = "Справа налево";
   } else if (current == "ltr") {
      input.attributes['dir'].value = "rtl"
        document.getElementById('change').value = "Слева направо";
   }
}
</script>
<textarea dir="ltr" id="textarea"></textarea><br />
<input type="button" id="change" onclick="changeDIR('textarea');" value="Справа налево">