multi-language always swapping back on page load

I am using the multi-language solution as given in Docs but unfortunately on new page load it reverts back to german even when english is selected. Here is the code I am using:

Session

session_start();
    $lang = 'de'; // default lang
    switch(perch_get('lang')) {
        case 'en':
            $lang = 'en';
            break;
        case 'de':
            $lang = 'de';
            break;
        // add more cases for language options here...

        default:
            if (isset($_SESSION['lang'])) {
                $lang = $_SESSION['lang'];
            }
            break;
    }
    $_SESSION['lang'] = $lang;

Example of on page code:

<?php if ($lang == 'en'){
            include("inc/lang_en.php");
            } else {
                include("inc/lang_de.php");
            }
        ?>

Anybody had this problem...?