My translation don’t work

Hello, sorry for my english for start ;)
I’ve got a problem with translation. I have read your instructions for other people and followed your instructions:
*created child theme
* added this code to my functions.php

function portal_child_set_text_domain(){
load_theme_textdomain( 'portal', get_stylesheet_directory() . '/languages' );
}
add_action('after_setup_theme', 'portal_child_set_text_domain', 15);

*translated using poedit and saved two files as pl_PL, added them to folder languages in my child theme folder
* checked if this code exist in my wp-config.php in my wordpress’ root

define('WPLANG', 'en_US');

Unfortunately my translation still isn’t visible on my site. What more can I do?
Thanks in advance for your help!