# views/layouts/application.html.erb
<% flash.each do |name, msg| -%>
  <%= content_tag :div, msg, class: name %>
<% end -%>

# app/controllers/index_controller.rb
def index
  flash[:notice] = t("Some notice")
end

# config/locales/en.yml
en:
  "Some notice": "Some Translated Notice"