Código HTML:
<select name="idioma" onchange="url=location.href.toString(); location.href=url.replace('&idioma=<? echo $_GET["idioma"];?>','&idioma='+this.options[this.selectedIndex].value)"> <option value="en"<? if($_GET["idioma"]=="en"){?> selected="selected"<? }?>>Ingles</option> <option value="de"<? if($_GET["idioma"]=="de"){?> selected="selected"<? }?>>Aleman</option> </select>