Loads plugin and theme text domains just-in-time.

When a textdomain is encountered for the first time, we try to load
the translation file from `wp-content/languages`, removing the need
to call load_plugin_textdomain() or load_theme_textdomain().

function _load_textdomain_just_in_time( $domain ) {
	/** @var WP_Textdomain_Registry $wp_textdomain_registry */
	global $l10n_unloaded, $wp_textdomain_registry;

	$l10n_unloaded = (array) $l10n_unloaded;

	// Short-circuit if domain is 'default' which is reserved for core.
	if ( 'default' === $domain || isset( $l10n_unloaded[ $domain ] ) ) {
		return false;
	}

	if ( ! $wp_textdomain_registry->has( $domain ) ) {
		return false;
	}

	$locale = determine_locale();
	$path   = $wp_textdomain_registry->get( $domain, $locale );
	if ( ! $path ) {
		return false;
	}
	// Themes with their language directory outside of WP_LANG_DIR have a different file name.
	$template_directory   = trailingslashit( get_template_directory() );
	$stylesheet_directory = trailingslashit( get_stylesheet_directory() );
	if ( str_starts_with( $path, $template_directory ) || str_starts_with( $path, $stylesheet_directory ) ) {
		$mofile = "{$path}{$locale}.mo";
	} else {
		$mofile = "{$path}{$domain}-{$locale}.mo";
	}

	return load_textdomain( $domain, $mofile, $locale );
}