i18n.rb

module I18n
  extend ActiveSupport::Concern

  included do
    attr_accessor :forced_locale
  end

  module ClassMethods

    def i18n_field(*syms)

      syms.each do |sym|
        class_eval(<<-EOS, __FILE__, __LINE__ + 1)

          def #{sym}
            self.#{sym}_translations[@forced_locale.to_s] || super
          end

          def #{sym}=(v)
            if @forced_locale
              self.#{sym}_translations = (self.#{sym}_translations || {}).merge(@forced_locale.to_s => v)
            else
              super(v)
            end
          end

        EOS

      end
    end
  end
end

post.rb

class Post
  include Mongoid::Document
  field :title, localize: true
  field :subtitle, localize: true

  include I18n
  i18n_field :title, :subtitle

end

post_spec.rb

require 'spec_helper'

describe Post do

  let(:post) do
    Post.create!
  end

  subject { post }

  context "when using hash" do

    before do
      post.title_translations = {'en' => 'hello', 'fr' => 'merci'}
      post.save!
      post.reload
    end

    it do
      post.reload.title_translations.should eq({'en' => 'hello', 'fr' => 'merci'})
    end
  end

  context "when using title property" do

    before do
      post.forced_locale = 'en'
      post.title = 'hello'
      post.forced_locale = 'fr'
      post.title ='merci'
    end

    its "title_translations" do
      should eq({'en' => 'hello', 'fr' => 'merci'})
    end

    it do
      post.save!
      post.reload
      post.title_translations.should eq({'en' => 'hello', 'fr' => 'merci'})
    end
  end
end