X theme is translatable and it only displays the strings provided by the System (which is Wordpress and translation plugin if available).
Wordpress and translation plugins uses the same or similar code to the one you're adding in functions.php (PHP), the difference is just you're directly supplying your preferred strings with that code while Wordpress and translation plugins using language files or database for the string lookup table.
And since the code added to the functions.php works means the translation works, except, maybe something in Wordpress or translation plugins is failing.
Would you mind providing your translated language files (po file)? I'll check and test it on my installation.