You can easily find missing translations with I18n::MissingTranslations gem.
Add this to Gemfile:
And this to the application config:
config.app_middleware.use(I18n::MissingTranslations) unless Rails.env.production?
Now browse your application or run integration tests and voila:
config/locales/missing_translations.yml contains all the missing keys.
It is also a nice way to create a translation file from scratch: specify your main traslation file as the destination for missing translations
and develop your views not worrying about the translations file - it will be created for you.