compare_lang_files.rb

require 'yaml'

def main
  lang_file_1 = ARGV[0]
  lang_file_2 = ARGV[1]

  lang_1 = YAML::load(File.open lang_file_1)
  lang_2 = YAML::load(File.open lang_file_2)

  lang_1_keys = all_keys lang_1[lang_1.keys.first]
  lang_2_keys = all_keys lang_2[lang_2.keys.first]

  a_not_in_b = lang_1_keys - lang_2_keys
  b_not_in_a = lang_2_keys - lang_1_keys

  if a_not_in_b.empty? && b_not_in_a.empty?
    puts "============================"
    puts "Both files contain same keys"
    puts "============================"
  else
    puts "Keys exist in the first file, but not in the second one"
    puts "========================================================"
    puts a_not_in_b

    puts "========================================================"
    puts "\n\n"
    puts "========================================================"
    puts "Keys exist in the second file, but not in the first one"
    puts "========================================================"
    puts b_not_in_a
  end
end

def all_keys hash, parent = ""
  keys = []
  hash.each do |key, value|
    if value.is_a? Hash
      keys += all_keys(value, [parent, key].select{|i| i unless i == ""}.join("."))
    else
      keys << [parent, key].select{|i| i unless i == ""}.join(".")
    end
  end

  keys
end

main

Usage:

ruby compare_lang_files.rb /path/to/config/locales/en.yml /path/to/config/locales/de.yml

Output are only the keys (no values). For example with both keys and values check out https://gist.github.com/bajalovic/75fac374ef57a1cbbe25