I18n Rails I18n

I tried to translate the errors message header and body and it seems that the interpolation didn't work properly.
here is what I did in 'error_messages_helper.rb' file (I use nifty_generators gem):
[code] module ErrorMessagesHelper
# Render error messages for the given objects. The :message and :header_message options are allowed. def error_messages_for(*objects) options = objects.extract_options!
options[:header_message] ||= t("activerecord.errors.template.header.other") options[:message] ||= t("activerecord.errors.template.body") messages = objects.compact.map { |o| o.errors.full_messages }.flatten unless messages.empty? content_tag(:div, :class => "error_messages") do list_items = messages.map { |msg| content_tag(:li, msg) } content_tag(:h2, options[:header_message]) + content_tag(:p, options[:message]) + content_tag(:ul, list_items.join.html_safe) end end end

module FormBuilderAdditions def error_messages(options = {}) @template.error_messages_for(@object, options) end end end

ActionView::Helpers::FormBuilder.send(:include, ErrorMessagesHelper::FormBuilderAdditions)
[/code] When generating a validation error, I got:
[code] "Impossible d'enregistrer %{model} : %{count} erreurs" [/code] inteads of the translated model name and count value.
I also attache the translation 'yml' file.