Add Corsican localization (#7390)

* Added Corsican localization

* Declaring Corsican localization

* Fixed activerecord locale

* Added a missing translation and switched to French-style apostrophes

* Fixed a (predictable) mistake made while bulk-replacing apostrophes

* More fixing

* i18n-tasks normalize

app/helpers/settings_helper.rb

@@ -6,6 +6,7 @@ module SettingsHelper
     ar: 'العربية',
     bg: 'Български',
     ca: 'Català',
+    co: 'Corsu',
     de: 'Deutsch',
     el: 'Ελληνικά',
     eo: 'Esperanto',