Validate both the model and its translations

This way we guarantee there will be at least one translation for a model
and we keep compatibility with the rest of the application, which
ideally isn't aware of globalize.

app/models/concerns/globalizable.rb

@@ -5,4 +5,11 @@ module Globalizable
     globalize_accessors
     accepts_nested_attributes_for :translations, allow_destroy: true
   end
+
+  class_methods do
+    def validates_translation(method, options = {})
+      validates(method, options.merge(if: lambda { |resource| resource.translations.blank? }))
+      translation_class.instance_eval { validates method, options }
+    end
+  end
 end