Vamos al grano
Código PHP:
<td class="Estilo3"><input name="idioma" type="radio" value="1" onClick="this.form.action='<? echo $PHP_SELF; ?>'?idioma=1, this.form.submit()" <? if( isset($_SESSION['idioma']) && $_SESSION['idioma']==1){echo "CHECKED";} ?>>Castellano</td>
<td class="Estilo3"><input name="idioma" type="radio" value="2" onClick="this.form.action='<? echo $PHP_SELF; ?>?idioma=2', this.form.submit()" <? if( isset($_SESSION['idioma']) && $_SESSION['idioma']==2){echo "CHECKED";} ?>>Catalán</td>