требует установленного гема ya2yaml (gem install ya2yaml)

en.yml

en:
  name: 
    first: Andrey
    last: Samsonov
  email: '[email protected]'
  github: 'https://github.com/kryzhovnik'
  phone: '+7(927)695-8-444'
  skype: 'andrey.samsonov'
  country: Russia
  city: Samara
  birthday:
    day: 16
    month: apr

locales_merger.rb

#!/usr/bin/env ruby
require 'yaml'
require 'ya2yaml'
  
class Hash
  def deep_merge(other_hash)
    dup.deep_merge!(other_hash)
  end
  
  def deep_merge!(other_hash)
    other_hash.each_pair do |k,v|
      tv = self[k]
      
      self[k] = if v.is_a?(Hash)
        tv.is_a?(Hash) ? tv.deep_merge(v) : self[k] = v
      else
        tv.nil? ? v : self[k]
      end
    end
    self
  end
end

if ARGV.size == 2 or ARGV.size == 3
  ARGV[2] ||= ARGV[1]
  complete_path, incomplete_path, result_path = ARGV[0..2]

  complete_hash   = YAML.load(File.open(complete_path))
  incomplete_hash = YAML.load(File.open(incomplete_path))

  # {:ru => { ... }}.first[1] == { ... } => true
  incomplete_hash.first[1].deep_merge!(complete_hash.first[1])

  File.open(result_path, 'w'){|f| f << incomplete_hash.ya2yaml}
else
  puts <<END 

Usage: lacales_merger COMPLETE_LOCALE_PATH INCOMPLETE_LOCALE_PATH [RESULT_PATH]
  if RESULT_PATH is missing, results writes to INCOMPLETE_LOCALE_PATH file
Example:
  ./locales_merger.rb fixtures/en.yml fixtures/ru.yml fixtures/result.yml
END
end

ru.yml

ru:
  name: Андрей Самсонов
  city: Самара
  country: Россия
  birthday:
    year: 1986
    month: 'апрель'