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'])) {
            if(!is_404()) {
              wp_redirect(currentURL().$_SESSION['lang'],301);
              exit;
            }
    	}
    }
    add_action('wp_head','lang_session');

    function output_buffer() {ob_start();}
    add_action('init','output_buffer');
    
    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