Puedes usar el caso 'default'
Código PHP:
if(!$_SESSION['language']){
$sitelang = getenv("HTTP_ACCEPT_LANGUAGE");
$_SESSION['language'] = substr($sitelang, 0, 2);
switch($_SESSION['language']){
case "es" :
include("templates/index.es");
break;
//case "en" :
default:
include("templates/index.en");
// break;
}
}else{
switch($_SESSION['language']){
case "es" :
include("templates/index.es");
break;
//case "en" :
default:
include("templates/index.en");
// break;
}
}