set plurals for and credit Estonian translation

lib/I18n.php

@@ -305,7 +305,7 @@ class I18n
     /**
      * determines the plural form to use based on current language and given number
      *
-     * From: http://localization-guide.readthedocs.org/en/latest/l10n/pluralforms.html
+     * From: https://localization-guide.readthedocs.org/en/latest/l10n/pluralforms.html
      *
      * @access protected
      * @static
@@ -334,7 +334,7 @@ class I18n
                 return $n % 10 == 1 && $n % 100 != 11 ? 0 : ($n % 10 >= 2 && $n % 10 <= 4 && ($n % 100 < 10 || $n % 100 >= 20) ? 1 : 2);
             case 'sl':
                 return $n % 100 == 1 ? 1 : ($n % 100 == 2 ? 2 : ($n % 100 == 3 || $n % 100 == 4 ? 3 : 0));
-            // bg, ca, de, en, es, hu, it, nl, no, pt
+            // bg, ca, de, en, es, et, hu, it, nl, no, pt
             default:
                 return $n != 1 ? 1 : 0;
         }