function checkEmail(str) {
	if ((str.indexOf(".") > 0) && (str.indexOf("@") > 0)) {
		error = "";
	} else {
		error = html_entity_decode( newsletter_langtr["check_email"]) + "\n";
	}
	return error;
}

function checkRegister() {
	var why = "";
	var email = document.registrati.email.value;
	var informativa = document.registrati.informativa.checked;	
	if( email == "") {
		why += html_entity_decode(newsletter_langtr["check_form"]) + "\n";
	}
	if( informativa == false) {
		why += html_entity_decode(newsletter_langtr["check_informativa"]) + "\n";
	}
    why += checkEmail(email);
    if (why != "") {
       alert(why);
	   return false;
    } else {
		document.registrati.submit();
		return false;
	}
}

1.
function html_entity_decode(str) {
	try {
		var tarea=document.createElement('textarea');
		tarea.innerHTML = str; return tarea.value;
		tarea.parentNode.removeChild(tarea);
	}
	catch(e)
	{
		//for IE add <div id="htmlconverter" style="display:none;"></div> to the page
		document.getElementById("htmlconverter").innerHTML = '<textarea id="innerConverter">' + str + '</textarea>';
		var content = document.getElementById("innerConverter").value;
		document.getElementById("htmlconverter").innerHTML = "";
		return content;
	}
}