ordering fields_for

agencies_controller.rb

def edit

  @agency = Agency.find(params[:id])
  # add comments for locales that don't have comments yet, so we can present fields to the user
  (I18n.available_locales.map(&:to_s) - @agency.comments.pluck(:locale)).each do |loc|
    @agency.comments.build locale: loc
  end
end

edit.html.haml

= simple_form_for @agency do |f|
  = f.simple_fields_for :comments do |c|
    = c.input :comment, label: "#{c.object.locale} comment:"