I18n Pluralization are useful when you want your application to customize pluralization rules.
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.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.