改变语言条件

Question

I try to make a 'change languge system' for my script. I already created my class and everything works fine. My problem is next: if I have URL index.php?lang=fr , my website language is changed as long I have ?lang=fr. If I remove ?lang=fr from URL, language is changed too back to english. What is wrong in my conditions ?

P.S: $available_langs is an array with en, fr and ro.

if(isset($_GET['lang']) && $_GET['lang'] != ''){
        if(in_array($_GET['lang'], $available_langs)){ 
            $_SESSION['lang'] = $_GET['lang'];
            $lang = new Language("languages/");
            $lang->setLang($_SESSION['lang']);
        } else {
            $_SESSION['lang'] = "en";
            $lang = new Language("languages/");
            $lang->setLang($_SESSION['lang']);
        }
    } else {
        if(isset($_SESSION['lang'])){
            $lang = new Language("languages/");
            $lang->setLang($_SESSION['lang']);
        } else {
            $_SESSION['lang'] = "en";
            $lang = new Language("languages/");
            $lang->setLang($_SESSION['lang']);      
        }
    }