I have po mo translation files, and I want to call my translation and use in my theme.
My theme name mytheme
I want to make my theme multilanguage, and make it by right way.
I created directory

mytheme/languages


I created my po mo files

mytheme/languages/ru_RU.po
mytheme/languages/ru_RU.mo


In wp-config.php file I add

define ('WPLANG', 'ru_RU');


In my header.php i call translation function

<?php echo __("Videos", "mytheme"); ?>


But it is not working, not translating.