Gemfile

gem 'deep_merge'

translations_simple_test.yml

'en.pets.types.cat': Cat
'en.pets.types.dog': Dog
'en.pets.title': My lovely pets
'en.actions.add': Add
'en.actions.remove': Remove
'en.language': <strong>Language</strong>

translations_simple_test_expected.yml

---
en:
  pets:
    types:
      cat: Cat
      dog: Dog
    title: My lovely pets
  actions:
    add: Add
    remove: Remove
  language: "<strong>Language</strong>"

yaml_converter.rb

require 'yaml'
require 'deep_merge'
require 'fileutils'

class YAMLConverter
  def initialize(filepath)
    @filepath = filepath
    @filepath_converted = "#{@filepath.split('.')[0..-2].join}_converted.yml"
  end

  def generate
    File.open(@filepath_converted, 'w') {|f| f.write converted }
  end

  def converted
    hash = {}
    YAML::load_file(@filepath).each do |key, value|
      new_hash = create_hash(key.split('.'), value)
      hash.deep_merge!(new_hash)
    end
    hash.to_yaml
  end

  private

  def create_hash(keys, value)
    hash = { "#{keys[-1]}" => value }

    keys.reverse[1..-1].each do |subkey|
      hash = { "#{subkey}" => hash }
    end
    hash
  end
end

class YAMLConverterTest
  class << self
    def passed?
      match('translations_simple_test.yml', 'translations_simple_test_converted.yml')
    end

    private

    def match(test_yml, expected_yml)
      YAMLConverter.new("#{Dir.pwd}/#{test_yml}").generate
      puts File.read("#{Dir.pwd}/translations_simple_test_converted.yml")
      puts ''
      result = FileUtils.compare_file('translations_simple_test_converted.yml', 'translations_simple_test_expected.yml')
      FileUtils.rm_f("#{Dir.pwd}/translations_simple_test_converted.yml")
      result
    end
  end
end

puts (YAMLConverterTest.passed? ? 'Tests passed successfully' : 'Tests passed with failure')