Monkey patching ActionMailer::Base#create! to support localized implicit multipart mails.

locale_rails_multipart_mail_patch.rb

# This is a FUGLY hack to make multipart emails work with localized templates
# in the form of method_el_GR.text.plain.rhtml (or .erb).
#
# NOTE : This is tested against Rails 2.3.8 and is a very good candidate to break in 2.3.9 and up.
ActionMailer::Base.class_eval do
  def create!(method_name, *parameters) #:nodoc:
    initialize_defaults(method_name)
    __send__(method_name, *parameters)

    # If an explicit, textual body has not been set, we check assumptions.
    unless String === @body
      # First, we look to see if there are any likely templates that match,
      # which include the content-type in their file name (i.e.,
      # "the_template_file.text.html.erb", etc.). Only do this if parts
      # have not already been specified manually.
      if @parts.empty?

        # XXX : Hacked part, no. 1
        [Dir.glob("#{template_path}/#{@template}_#{GetText.locale}.*"),
         Dir.glob("#{template_path}/#{@template}.*")].flatten.each do |path|
          template = template_root["#{mailer_name}/#{File.basename(path)}"]

          # XXX : Hacked part, no. 2
          # Skip unless template has a multipart format and the current content type is not already included
          content_type_not_already_included = @parts.select { |p| p.content_type == template.content_type }.blank?
          next unless template && template.multipart? && content_type_not_already_included

          @parts << ActionMailer::Part.new(
                  :content_type => template.content_type,
                  :disposition => "inline",
                  :charset => charset,
                  :body => render_message(template, @body)
          )
        end
        unless @parts.empty?
          @content_type = "multipart/alternative" if @content_type !~ /^multipart/
          @parts = sort_parts(@parts, @implicit_parts_order)
        end

      end

      # Then, if there were such templates, we check to see if we ought to
      # also render a "normal" template (without the content type). If a
      # normal template exists (or if there were no implicit parts) we render
      # it.
      template_exists = @parts.empty?
      # XXX : Hacked part, no. 3
      template_exists ||= template_root["#{mailer_name}/#{@template}_#{GetText.locale}"] || template_root["#{mailer_name}/#{@template}"]
      @body = render_message(@template, @body) if template_exists

      # Finally, if there are other message parts and a textual body exists,
      # we shift it onto the front of the parts and set the body to nil (so
      # that create_mail doesn't try to render it in addition to the parts).
      if !@parts.empty? && String === @body
        @parts.unshift ActionMailer::Part.new(:charset => charset, :body => @body)
        @body = nil
      end
    end

    # If this is a multipart e-mail add the mime_version if it is not
    # already set.
    @mime_version ||= "1.0" if !@parts.empty?

    # build the mail object itself
    @mail = create_mail
  end
end