Well, that’s very interesting.
/wp-includes/media.php is a WordPress core file. The undefined, is_rtl() function is also a WordPress core function. It looks like your WordPress install is damaged.
Line 1668 in inside the implementation of the WordPress [gallery] shortcode. It looks like the problem occurs when a [gallery] shortcode is being processed.
Have a look in your /wp-includes/locale.php file and make sure you see the function definition at the bottom of the file:
/**
* Checks if current locale is RTL.
*
* @since 3.0.0
*
* @global WP_Locale $wp_locale
*
* @return bool Whether locale is RTL.
*/
function is_rtl() {
global $wp_locale;
return $wp_locale->is_rtl();
}
I did find this interesting comment in a WordPress file that loads locale.php:
* Used for errors encountered during the initial loading process, before
* the locale has been properly detected and loaded.
*
* Designed for unusual load sequences (like setup-config.php) or for when
* the script will then terminate with an error, otherwise there is a risk
* that a file can be double-included.
When does this problem occur, i.e., what did you do just before it started?
I will try to reproduce this symptom, but at this point it’s hard to see how MLA is causing this specific problem. It could be an interaction with one of your other plugins, such as Wordfence Security or WP Super Cache. If you could take the time to briefly disable them it might yield a clue. I regret that I don’t have a better answer at this moment.
