title: Django Parler how to access translated model field from a mixin
body: <p>I have written this model.</p>
  
  <pre><code>class Course(TranslatableModel):
      translations = TranslatedFields(
          title = models.CharField(max_length=200),
          overview = models.TextField(),
          slug = models.SlugField(max_length=200, unique=True))
      owner = models.ForeignKey(User, related_name='courses_created')
      subject = models.ForeignKey(Subject, related_name='courses')
      created = models.DateTimeField(auto_now_add=True)
      order = OrderField(blank=True, for_fields=['title'])
  
      class Meta:
          ordering = ('order',)
  
      def __unicode__(self):
          return self.title
  </code></pre>
  
  <p>AND ALSO THIS mixin class </p>
  
  <pre><code>class OwnerCourseEditMixin(OwnerCourseMixin, OwnerEditMixin):
      fields = ['subject', 'title', 'slug', 'overview']
      success_url = reverse_lazy('manage_course_list')
      template_name = 'courses/manage/course/form.html'
  </code></pre>
  
  <p>The "fields = ['subject', 'title', 'slug', 'overview']" line is causing the error </p>
  
  <p>Exception Type: FieldError
  Exception Value: Unknown field(s) (overview, slug, title) specified for Course</p>
  
  <p>How do I refer to the translated fields? If I remove 'title', 'slug', 'overview' from the fields list it works. </p>