add basic RTL support, drop default language key

lib/I18n.php

@@ -272,6 +272,18 @@ class I18n
         return array_intersect_key(self::$_languageLabels, array_flip($languages));
     }
 
+    /**
+     * determines if the current language is written right-to-left (RTL)
+     *
+     * @access public
+     * @static
+     * @return bool
+     */
+    public static function isRtl()
+    {
+        return in_array(self::$_language, array('ar', 'he'));
+    }
+
     /**
      * set the default language
      *