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I got it now , After searched many resources :

function lang_support() {
    return array('en','fr'); // Add your support lang-code (1st place is a default)
}

function rewrite_lang(){
    $langs = lang_support();
    foreach($langs as $lang) {
        add_rewrite_endpoint($lang,EP_PERMALINK|EP_PAGES|EP_ROOT|EP_CATEGORIES);
    }
}
add_action('init','rewrite_lang');

function lang(){
    global $wp_query;
    $langs = lang_support();
    $lang_r = "";
    foreach($langs as $lang) {
        if(isset($wp_query->query_vars[$lang])) {
            $lang_r = $lang;
            $_SESSION['lang'] = $lang_r;
        }
    }
    if(in_array($lang_r,$langs)) {
        return $lang_r;
    } else {
        return $langs[0];
    }
}

function init_session(){session_start();}
add_action('init','init_session',1);

function lang_session() { // Redirect by JS if session is set
    $url_lang= basename($_SERVER['REQUEST_URI']);
    if(!in_array($url_lang,lang_support()) && isset($_SESSION['lang'])) {
        echo'<script>window.location="'.currentURL().$_SESSION['lang'].'";</script>';
    end();
    }
}
add_action('wp_head','lang_session');

function currentURL() {
    $pageURL=(@$_SERVER["HTTPS"]=="on")?"https://":"http://";
        if($_SERVER["SERVER_PORT"]!="80"){
            $pageURL.=$_SERVER["SERVER_NAME"].":".$_SERVER["SERVER_PORT"].$_SERVER["REQUEST_URI"];
        }else{
            $pageURL.=$_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"];
        }
        return $pageURL;
}

After added code go to wp-admin/options-permalink.php press Save Changes

So now you can check/use lang code in your template like..

<?php echo lang();?>

or if( lang() === "en" )

Example work urls with my example code :

http://domain.com/ <-- "en"
http://domain.com/en/ <-- "en"
http://domain.com/fr/ <-- "fr"
http://domain.com/es/ <-- "en" (coz , "es" code not set)

http://domain.com/mypage/ <-- "en" (work with "page")
http://domain.com/mypage/fr/ <-- "fr"
http://domain.com/mypage/es/ <-- "en" (coz , "es" code not set)

http://domain.com/mypost/ <-- "en" (work with "post")
http://domain.com/mypost/fr/ <-- "fr"
http://domain.com/mypost/es/ <-- "en" (coz , "es" code not set)

Better idea / bug fixer / found error + bug ? https://gist.github.com/l2aelba/5244912