I got it now , After searched many resources :

    add_action('init','rewrite_lang');
    function rewrite_lang(){
    	$langs = array('en','fr'); // Set your langs here
    	foreach($langs as $lang) {
    		add_rewrite_endpoint($lang,EP_PERMALINK|EP_PAGES|EP_ROOT);
    	}
    }
    
    function lang(){
        global $wp_query;
        $langs = array('en','fr'); // Set your langs here too
        $lang_r = "";
        foreach($langs as $lang) {
            if(isset($wp_query->query_vars[$lang])) {
            	$lang_r = $lang;
            }
        }
    	if(in_array($lang_r,$langs)) {
        	return $lang_r;
    	} else {
    		return "en"; // If lang not set in url so default is "en"
    	}
    }



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)