Source

File: wp-includes/l10n.php

function _get_path_to_translation_from_lang_dir( $domain ) {
	static $cached_mofiles = null;

	if ( null === $cached_mofiles ) {
		$cached_mofiles = array();

		$locations = array(
			WP_LANG_DIR . '/plugins',
			WP_LANG_DIR . '/themes',
		);

		foreach ( $locations as $location ) {
			$mofiles = glob( $location . '/*.mo' );
			if ( $mofiles ) {
				$cached_mofiles = array_merge( $cached_mofiles, $mofiles );
			}
		}
	}

	$locale = determine_locale();
	$mofile = "{$domain}-{$locale}.mo";

	$path = WP_LANG_DIR . '/plugins/' . $mofile;
	if ( in_array( $path, $cached_mofiles, true ) ) {
		return $path;
	}

	$path = WP_LANG_DIR . '/themes/' . $mofile;
	if ( in_array( $path, $cached_mofiles, true ) ) {
		return $path;
	}

	return false;
}