error_backend.rb

module I18n
  module Backend
    class Error
      include Base

      def lookup(locale, key, scope = [], options = {})
        case key
        when /missing/
          nil
        when /count/
          { :one => "one" }
        end
      end
    end
  end
end

I18n.backend = I18n::Backend::Error.new
I18n.exception_handler = ->(*args) { p args }
I18n.t('missing.something') # Outputs args from handler as excepted
I18n.t('count.something', :count => 2) # Raises exception