I loved this code. Unfortunately I can't use it anymore. May it rest in peace here.

collect_translations.rake

require 'fileutils'

namespace :i18n do
  desc 'collect all translation keys from Ruby/HAML files and fill translations table with them'
  task :collect_translations => :environment do

    def each_translation_key
      return to_enum(__method__) unless block_given?                  # Make this method into an enumerator.
      Dir.glob(Rails.root.join('app', '**', '*.{rb,haml}')) do |path| # Rails.root is a Pathname!
        File.read(path).scan(/_\(['"](.*?)['"]\)/) do |result|        # Because of the (.*) group,
          yield result.first                                          # .scan will return a 1-item array!
        end
      end
    end

    each_translation_key do |key|
      I18n.available_locales.each do |locale|
        # I18n::Backend::ActiveRecord::* does NOT behave like a normal AR object!
        begin 
          t = Translation.where(key: key, locale: locale).first!
          puts "#{locale}: [#{key}] exists"
        rescue
          t = Translation.new(key: key, locale: locale)
          puts "#{locale}: creating [#{key}]"
        end
        # The sources contain English strings, no matter what I18n.default_locale is.
        t.value = key if locale == :en && t.value.blank? 
        t.save # If it already existed, a value might have been added.
      end
    end

  end
end