i18n missing keys to gengo export/import

translator.rake

namespace :translator do
  desc "Export missing translations for a specific locale"
  task :export_keys => :environment do
    from = ENV['FROM']
    to   = ENV['TO']
    if from.present? and to.present?
      translator = Translator.new(
        from: from.to_sym,
        to:   to.to_sym
      )
      missing = translator.export_keys
      if missing.length > 0
        filename = "translate_#{from}_to_#{to}.txt"
        File.open(filename, 'w') do |file|
          file.write missing
        end
        puts "Created export file: #{filename}"
      else
        puts "There are no missing translations from #{from} to #{to}"
      end
    else
      puts "Please provide locale to translate for example:"
      puts "rake translator FROM=en TO=fr"
    end
  end

  desc "Import missing translations for a specific locale"
  task :import_keys => :environment do
    if ENV['FROM'].present? and ENV['TO'].present? and ENV['FILE'].present?
      translator = Translator.new(
        from: ENV['FROM'].to_sym,
        to:   ENV['TO'].to_sym
      )
      file = File.open(ENV['FILE'])
      translator.import_keys(file.read)
      translator.write_locale_file
    else
      puts "Please provide the following arguments:"
      puts "rake translator FROM=en TO=fr FILE=en_to_fr.translate"
    end
  end
end

translator.rb

require 'yaml'

class Translator
  def initialize(options = {})
    @from = options[:from]
    @to   = options[:to]
    @dir  = options[:dir] || "config/locales/multilingual"
  end

  def prepare_translations_for_missing_keys
    I18n.with_locale(@from) do
      result = {}
      find_missing_keys.each do |key|
        result[key] = I18n.t(key.gsub("#{@to}.", '')).to_s
      end
      result
    end
  end

  def export_keys
    prepare_translations_for_missing_keys.map do |key, value|
      "[[[#{key}]]] #{value}"
    end.join("\n")
  end

  def import_keys(import)
    matches = import.scan %r{
      \[\[\[           # key start brackets
      ([^\]]+)         # key
      \]\]\]           # key end brackets
      ((.(?!\[\[\[))*) # value until brackets
    }xm
    @import = {}
    matches.each do |match|
      @import[match[0]] = match[1].to_s.lstrip
    end
    @import
  end

  def write_locale_file
    old_yaml    = yaml(@to)
    new_yaml    = deflatten_keys(@import)
    merged_yaml = old_yaml ? old_yaml.deep_merge(new_yaml) : new_yaml
    File.open(path(@to), 'w') do |file|
      file.write merged_yaml.to_yaml
    end
  end

  def find_missing_keys
    yaml_1 = yaml(@from)
    yaml_2 = yaml(@to)
    keys_1 = yaml_1.present? ? flatten_keys(yaml_1[yaml_1.keys.first]) : []
    keys_2 = yaml_2.present? ? flatten_keys(yaml_2[yaml_2.keys.first]) : []
    (keys_1 - keys_2).map {|k| "#{@to}.#{k}" }
  end

private

  def yaml(locale)
    YAML.load((File.open(path(locale)) rescue ''))
  end

  def path(locale)
    File.expand_path("#{@dir}/#{locale}.yml")
  end

  def deflatten_keys(hash)
    new_hash = {}
    hash.each do |k,v|
      new_hash.deep_merge!(
        k.split('.').reverse.inject(v) {|a,n| { n => a } }
      )
    end
    new_hash
  end

  def flatten_keys(hash, prefix="")
    keys = []
    hash.keys.each do |key|
      if hash[key].is_a? Hash
        current_prefix = prefix + "#{key}."
        keys << flatten_keys(hash[key], current_prefix)
      else
        keys << "#{prefix}#{key}"
      end
    end
    prefix == "" ? keys.flatten : keys
  end
end

FYI, I made a few changes in my fork. In my 'from' or starting locale I generally break things into multiple files for easier maintenance, so I load all translations differently and then pick that locale instead of just loading from a single file. Thanks again!