While having this form wtih a language type and custom choice type, i noticed submitting an unknown value triggers a transformation value (thus form error) on the choice field but not for the language field.

This worked OK in 2.7

$form = $this->createFormBuilder(null, array('csrf_protection' => false)))
    ->add('a', LanguageType::class)
    ->add('b', ChoiceType::class, array('choices' => array(1,2,3)))
    ->getForm();
$form->handleRequest($request);

POST /
form[a]=x&form[b]=x

3.2

2.7