Hola bhonox.
Prueba esto:
Código PHP:
<html>
<head>
<script language="JavaScript">
function showHide(bChk){
var form = document.frm;
if(!bChk){
form.drop.style.visibility = 'hidden';
} else {
form.drop.style.visibility = 'visible';
}
return true;
}
</script>
</head>
<form action="" method="post" name="frm">
<input type="radio" name="C1" checked onClick="return showHide(false)">
<font face="Arial" size="1">Español</font>
<input type="radio" name="C1" onClick="return showHide(false)">
<font face="Arial" size="1">Inglés</font>
<input type="radio" name="C1" onClick="return showHide(true)">
<font face="Arial" size="1">Otro (especifique)</font><br>
<select name="drop" size="1" style="visibility: hidden">
<option value="" selected>[SELECT]</option>
<option value="Uno">ASCII</option>
<option value="Dos">Binario</option>
<option value="Dos">Unicode</option>
</select><br>
</form>
</body>
</html>
Saludos.