Is it possible to localize the button captions of dialogs without
editing the js-file?

It seems to be here:

function webctrls_msgdlgbtntostr(value)
{
   var $r;
   if (value == webctrls_mbok)
      $r = "OK";
   else if (value == webctrls_mbcancel)
      $r = "Cancel";

Thanks Uli