example_model.rb

class Example
  include MongoMapper::Document
  key :title, LocalizedString
end

gistfile1.rb

I18n.locale = :en
e = Example.new
e.title << "English Title"
I18n.locale = :hy 
e.title << "Armenian Title" # data added after changing the locale is saved for that locale
puts e.title # returns data for the current locale (i.e. "Armenian Title")
I18n.locale = :en
puts e.title # returns "English Title"
puts e.title[:hy] # translations can also be explicitly called

LocalizedString.rb

class LocalizedString < HashWithIndifferentAccess
  def self.from_mongo(value)
    value.is_a?(String) ? value : LocalizedString.new(value || {})
  end

  def available_locales
    symbolize_keys.keys
  end

  def to_s
    self[I18n.locale]
  end

  def in_current_locale=(value)
    self[I18n.locale] = value
  end

  def << (value)
   self[I18n.locale] = value
  end
end