According to http://blogs.jetbrains.com/ruby/2009/08/whats-mining-rails-i18n-support/ there should be a quick fix menu that allows me to create translation keys. It seems to be related to the light bulb but in the latest RubyMine (RM-96.886) I don't seem to get the little lightbulb. Any ideas how to get to that menu?
Make sure you have 'i18n string value' intention enabled in IDE Settings | Intentions | Ruby. When you're on a string in the code try Alt+Enter (Option+Enter on Mac). It should show the quick-fix popup.