I18n locale fallbacks are useful when you want your application to use translations from other locales when translations for the current locale are missing. E.g. you might want to use :en translations when translations in your applications main locale :de are missing.

To enable locale specific pluralizations you can simply include the Pluralization module to the Simple backend - or whatever other backend you are using.

I18n::Backend::Simple.send(:include, I18n::Backend::Pluralization)

You also need to make sure to provide pluralization algorithms to the backend, i.e. include them to your I18n.load_path accordingly.