I don't know if this would work or not, but it's sure worth a try.
What if you added something like this to the bottom of your pixelpost.php file (located in the includes directory:
PHP Code:
if (isset($_COOKIE['jfcookie'])) {
// If the jfcookie is set, tell Pixelpost to use the same langauge:
$_GET['lang'] = $_COOKIE['jfcookie']['lang'];
}elseif(isset($_COOKIE['lang'])) {
// Otherwise, if the pixelpost langauge is set, set the cookie for jfcookie:
setcookie ('jfcookie[\'lang\']', substr($_COOKIE['lang'],0,2), time() +60*60*24*30, '/', false, 0);
}
Note: this is un-tested, and might need a bit of tweaking, as I don't know exactly how your Joomla cookie system is setup.