public function isRtl( $lang = null ) {
		static $rtlLanguages = null;

		$lang = $lang ? $this->normalizeLang( $lang ) : $this->getLang();

		if ( $rtlLanguages === null ) {
			$file = $this->localBaseDir . '/language/rtl.json';
			$rtlLanguages = json_decode( file_get_contents( $file ), true );
		}

		return in_array( $lang, $rtlLanguages );
	}