change_lang_of_epub.rb

require "rubygems"
require "zipruby" # gem install zipruby
require "rexml/document"
require "fileutils"

class Zip::Archive
  def index
    i = 0
    each do |f|
      if yield f
        return i
      end
      i += 1
    end
    nil
  end
end

unless filename = ARGV.shift
  puts "File name required. usage: ruby change_lang_of_epub.rb somebook.epub"
  exit
end

lang = ARGV.shift || "ja"

new_filename = filename.dup
new_filename[-File.extname(new_filename).length, 0] = ".#{lang}"

FileUtils.cp filename, new_filename

Zip::Archive.open(new_filename) do |ar|
  unless opf_index = ar.index{|f| f.name =~ /\.opf\z/}
    puts "OPF not found"
    exit
  end

  doc = nil
  ar.fopen(opf_index) do |f|
    doc = REXML::Document.new(f.read)
  end

  doc.elements["/package/metadata/dc:language"].text = lang

  ar.replace_buffer(opf_index, doc.to_s)
end

puts "done"