A Ruby on Rails initializer which binds I18n.translate and logs every lookup to logs/i18n_registry.log. Tested on Rails 3.

i18n_registry.rb

# adapted from: http://stackoverflow.com/a/10211540/514700
module I18n
  module Registry
    protected
    def lookup(locale, key, scope = [], options = {})
      @log ||= Logger.new(File.join(Rails.root, 'log', 'i18n_registry.log'))
      @log.info "i18n lookup - key: #{key} | locale: #{locale} | scope: #{scope}"
      super
    end
  end
end

I18n::Backend::Simple.send :include, I18n::Registry