Add Polish as a supported language

We already had some Polish translations, that were currently not
available to users. Adding it here because it helps me debug some
issues regarding language with more than one form.

lib/gitlab/i18n.rb

@@ -21,7 +21,8 @@ module Gitlab
       'nl_NL' => 'Nederlands',
       'tr_TR' => 'Türkçe',
       'id_ID' => 'Bahasa Indonesia',
-      'fil_PH' => 'Filipino'
+      'fil_PH' => 'Filipino',
+      'pl_PL' => 'Polski'
     }.freeze
 
     def available_locales