With Symfony 1.2 the creation of I18n admin interface has changed.
It's very simple: if you have an Article model, just add to your ArticleForm:

class ArticleForm extends BaseArticleForm
{
  public function configure()
  {
    $this->embedI18n(array('en', 'fr'));
  }
}

and you can change the language labels with:

    $this->widgetSchema->setLabel('en', 'English');
    $this->widgetSchema->setLabel('fr', 'French');

for reference take a look at Form I18n