function inlocuire_diacritice (form) {
var form = event.target;
var i, l;
for (i = 0, l = form.elements.length; i < l; i += 1) {
if (form.elements[i].type === 'text') {
  form.elements[i].value = form.elements[i].value.replace(/Ş/g, 'Ș');
  form.elements[i].value = form.elements[i].value.replace(/ş/g, 'ș');
  form.elements[i].value = form.elements[i].value.replace(/Ţ/g, 'Ț');
  form.elements[i].value = form.elements[i].value.replace(/ţ/g, 'ț');
}
}
return true;
}

function validate(form) {
//form validation script
inlocuire_diacritice (form);
}

<form action="whatever" name="whatever" method="post" onsubmit="return validate(this)">

<form action="whatever" name="whatever" method="post" onsubmit="return validate(event, this)">...</form>

function inlocuire_diacritice (ev, form) {
    // form is a reference to the form element already
    // if you need event.target, use ev.target
    // this refers to the element to which this event is attached
                :
}