I ♥ Ruby so much...

000_create_categories.rb

class CreateCategories < ActiveRecord::Migration
  def self.up
    create_table :categories do |t|
      t.string :name_en,
               :name_cz,
               :name_es
      t.timestamps
    end
  end

  def self.down
    drop_table :categories
  end
end

000_create_products.rb

class CreateProducts < ActiveRecord::Migration
  def self.up
    create_table :products do |t|
      t.string   :name
      t.text     :description
      t.datetime :manufactured_on
      t.timestamps
    end
  end

  def self.down
    drop_table :products
  end
end

000_create_translations.rb

class CreateTranslations < ActiveRecord::Migration
  def self.up
    create_table :translations do |t|
      t.string  :name
      t.text    :description
                
      t.string  :locale
                
      t.string  :translatable_type
      t.integer :translatable_id

      t.timestamps
    end
    add_index :translations, [:locale, :translatable_id, :translatable_type], :name => "lang_index", :unique => true
  end

  def self.down
    drop_table :translations
  end
end

category.rb

class Category < ActiveRecord::Base

  I18n.backend.send(:init_translations) if I18n.available_locales.blank?

  self.class_eval do

    Category.columns_hash.collect { |c| c[0] if c[1].type == :string or c[1].type == :text }.compact.each do |attribute_name|

      puts attribute_name

      if includes_locale = I18n.available_locales.find { |locale| attribute_name =~ Regexp.new("_#{locale}$")  }

        method_name = attribute_name.sub( Regexp.new("(\w*)_#{includes_locale}$"), '\1' )

        unless method_defined?(method_name.to_sym)

          # Use like this:
          # c = Category.first
          # c.name_en
          # => "Category one"
          # c.name
          # => "Category one"
          # I18n.locale = 'es'
          # c.name
          # => "Categoria uno"
          puts "Defining localized getter '#{method_name}'"
          define_method(method_name) do |*args|
            lookup_locale = args.shift || I18n.locale
            puts "Arguments: #{args.inspect}"
            puts "Lookup locale: #{lookup_locale}"
            return @attributes["#{method_name}_#{lookup_locale}"]
          end

          # And now try this
          # c = Category.first
          # I18n.locale = 'es'
          # c.name
          # => "Categoria uno"
          # c.name = 'Pues, vamos a modificar este registro!'
          # c.save
          # >> UPDATE `categories` SET `name_es` = 'Pues, vamos a modificar este registro!' ... WHERE ...
          puts "Defining localized setter '#{method_name}='"
          define_method("#{method_name}=") do |args|
            raise ArgumentError,
                  "argument must be a String, like 'Salut' or a hash of values, like :es => '¡Hola!'" unless args.is_a?(String) || args.is_a?(Hash)
            locale = args.is_a?(Hash) ? args.keys.first : I18n.locale
            value  = args.is_a?(Hash) ? args.values.first : args
            puts "Arguments: #{args.inspect}"
            puts "Lookup locale: #{locale}"
            self.send(:"#{method_name}_#{locale}=", value) if self.respond_to?(:"#{method_name}_#{locale}")
          end

        end
     end

    end
  end #:nodoc: class_eval

end #:nodoc: class

product.rb

class Product < ActiveRecord::Base

  # This is really it
  has_many :translations, :as => :translatable

  self.class_eval do
    
    Product.columns_hash.collect { |c| c[0] if c[1].type == :string or c[1].type == :text }.compact.each do |attribute_name|
      
      # Use like this:
      # p = Product.first
      # p.name
      # => "Product two"
      # I18n.locale = :cz
      # p.name
      # => "Produkt 2"
      # I18n.locale = :en
      # p.name(:cz)
      # => "Produkt 2"
      puts "Re-defining method '#{attribute_name}' for #{self.class.name}"
      define_method(attribute_name) do |*args|
        lookup_locale = args.shift || I18n.locale
        puts "Arguments: #{args.inspect}"
        puts "Lookup locale: #{lookup_locale}"
        return @attributes[attribute_name] if lookup_locale.to_s == I18n.default_locale.to_s # Do not lookup anything if passed locale is default
        if translation = self.translations.find_by_locale(lookup_locale.to_s)
          translation.attributes[attribute_name]
        end
      end
      # This is needed so Rails form helpers like f.text_field :name actually work. They call _before_type_cast. 
      # See http://github.com/rails/rails/tree/2-2-stable/actionpack%2Flib%2Faction_view%2Fhelpers%2Fform_helper.rb#L649-655
      alias :"#{attribute_name}_before_type_cast" :"#{attribute_name}"
      
      # And now try this:
      # p = Product.first
      # p.name
      # => "Product two"
      # p.name = 'This is product two'
      # p.save
      # p.reload.name
      # => "This is product two"
      # I18n.locale = 'cz'
      # p.name
      # => "Produkt 2"
      # p.name = 'Produkt dvě'
      # p.save
      # p.reload.name
      # => "Produkt dvě"
      # p.name={:es => 'Esto es un nombre de producto dos'}
      # p.save
      # >> INSERT INTO `translations` (`name` ... `locale` ...) VALUES('Esto es un nombre de producto dos' ... 'es' ...)
      # p.reload.name
      # => "Produkt dvě"
      # p.name(:es)
      # => "Esto es un nombre de producto dos"
      puts "Re-defining method '#{attribute_name}=(value)' #{self.class.name}"
      define_method("#{attribute_name}=") do |args|
        puts "Arguments: #{args.inspect}"
        raise ArgumentError,
              "argument must be a String, like 'Salut' or a hash of values, like :es => '¡Hola!'" unless args.is_a?(String) || args.is_a?(Hash)
        # TODO : Allow passing more locales and values at once, like this <tt>p.name={ :cz => 'Ahoj', :es => 'Hola' }</tt>
        locale = args.is_a?(Hash) ? args.keys.first : I18n.locale
        value  = args.is_a?(Hash) ? args.values.first : args
        puts "Write to locale: #{locale}"
        puts "Value: #{value}"
        super(value.to_s) and return if locale.to_s == I18n.default_locale.to_s
        raise  ActiveRecord::RecordNotSaved, "you cannot add translations to unsaved record" if new_record?
        if translation = self.translations.find_by_locale(locale.to_s)
          translation.update_attribute(attribute_name.to_sym, value.to_s)
        else
          self.translations.create!(:locale => locale.to_s, attribute_name.to_s => value.to_s)
        end
      end
      
    end
    
  end #:nodoc: class_eval

  # Returns true if object has translation for passed locale
  def has_translation?(locale)
    !translations.find_by_locale(locale.to_s).nil?
  end
  
end