// creating the custom constraint message

entity.name.not_blank: 'Bitte einen Namen eingeben!'

// creating the translator

$translator = new Translator('de');
$translator->setFallbackLocales(['de']);
$translator->addLoader('xlf', new XliffFileLoader());
$translator->addLoader('yml', new YamlFileLoader());

// adding default translations
$translator->addResource('xlf', "path/to/vendor/symfony/form/Resources/translations/validators.de.xlf", 'de', 'validators');
$translator->addResource('xlf', "path/to/validator/Resources/translations/validators.de.xlf", 'de', 'validators');

// adding custom translations
$translator->addResource('yml', "path/to/messages.de.yaml", 'de');
$translator->addResource('yml', "path/to/validators.de.yaml", 'de', 'validators');

…

// setting up Twig

$defaultFormTheme = 'bootstrap_4_layout.html.twig';
$appVariableReflection = new ReflectionClass('SymfonyBridgeTwigAppVariable');

$twig = new Environment(new FilesystemLoader(['/path/to/views', '/path/to/Resources/views/Form']));

$formEngine = new TwigRendererEngine([$defaultFormTheme], $twig);

$twig->addRuntimeLoader(
  new FactoryRuntimeLoader([
    FormRenderer::class => function () use($formEngine, $csrfManager) {
      return new FormRenderer($formEngine, $csrfManager);
    }
  ])
);

$twig->addExtension(new FormExtension());
$twig->addExtension(new TranslationExtension($translator));

…

// creating the form factory adding the Validator extension

$validator = Validation::createValidator();
$formFactory = Forms::createFormFactoryBuilder()
    ->addExtension(new ValidatorExtension($validator))
    ->getFormFactory();

…

// rendering the form using Twig

echo $this->getTwig()->render('form.html.twig', [
        'form' => $form->createView()
    ]);

// setting up the EntityType form

public function buildForm(FormBuilderInterface $builder, array $options) {
    $builder
      ->add('name', TextType::class, [
          'constraints' => [
              new NotBlank(['message' => 'entity.name.not_blank'])
          ]
      ]);
}