[Ervin]

(function(){
 var input,
      checkbox;
 
 function startFn() {
   input = document.getElementById('forum-devushek');
   checkbox = document.getElementById('mycheckbox');
   
   checkbox.onchange = handleChange;
   
   if(input.value === "Я согласна") {
    checkbox.checked = true;
   }
 }
 
 function handleChange() {
   input.value = this.checked ? "Я согласна" : "Я не согласна";
 }
 
 window.onload = startFn;
 })();