sentence.rb

class Sentence < ActiveRecord::Base

  has_many :translations
  
  accepts_nested_attributes_for :translations, :allow_destroy => true, :reject_if => proc { |obj| obj[:text].blank? }
  
end

sentence_spec.rb

require 'spec_helper'

describe Sentence do

  it "validates translation language" do
    s = Sentence.new({
      :language => 'en',
      :text => 'hello world',
      :translations_attributes => {
        "0" => {:language => 'it', :text => 'ciao mondo'},
        "1" => {:language => 'fr', :text => 'bonjour monde'}
      }
    })
    
    s.save
  end

end

## The test errors out when saving the record:
# => undefined method `language' for nil:NilClass

translation.rb

class Translation < ActiveRecord::Base

  belongs_to :sentence
  validate :disallow_same_language_translations
  
  def disallow_same_language_translations
    # self.sentence returns nil even if sentence 'accepts_nested_attributes_for :translations'
    if self.sentence.language == self.language
      errors.add(:language, 'You can\'t add a translation in the same language as the source')
    end
  end  
  
end