What is the Use of undefined constant WP_LANG_DIR error?

Question

WordPress

Notice: Use of undefined constant WP_LANG_DIR - assumed 'WP_LANG_DIR' in /home/savyc146/domains/example.ua/public_html/wp-includes/l10n.php on line 719

Error on Wordpress site Line 718-733 of l10n.php
file

spoiler

// Try to load from the languages directory first.
if ( load_textdomain( $domain, WP_LANG_DIR . '/plugins/' . $mofile ) ) {
return true;
}
if ( false !== $plugin_rel_path ) {
$path = WP_PLUGIN_DIR . '/' . trim( $plugin_rel_path, '/' );
} elseif ( false !== $deprecated ) {
_deprecated_argument( __FUNCTION__, '2.7.0' );
$path = ABSPATH . trim( $deprecated, '/' );
} else {
$path = WP_PLUGIN_DIR;
}
return load_textdomain( $domain, $path . '/' . $mofile );
}

Tell me how to fix it, I think because of this error 500 pops up when creating pages / posts / media and even menu items.
Thank you very much for your attention

Answer 1

https://github.com/WordPress/WordPress/blob/280aef...

Answer 2

In general, this constant is either manually set in wp-config.php (at an early stage of loading), if you need to change the standard path to translation files, or set by the engine later , in the wp-includes/load.php file, with the following value:
WordPress looks, if it was not defined in wp-config.php, then it defines itself. There shouldn't be any mistakes . If you have an error, then this, as it were, hints that your kernel boot process has been modified, something you are doing wrong. This error should not be.