Código PHP:
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method='GET'>
<select onchange="this.form.submit()" name="lng" id="lng">
<option <?php if ($_GET['lng']=='es') {echo"selected";}else{echo"";}?> value='es'>Español</option>
<option <?php if ($_GET['lng']=='en') {echo"selected";}else{echo"";}?> value='en'>English</option>
</select>
</form>
<?php
if ($_GET['lng']) {
$idioma=$_GET['lng'];
}
else
{
$idioma='es';
}
//include("language/$idioma.php");
echo("<script>$('#prueba').load('language/".$idioma.".php');</script>");
?>