It took me several hours to make it, Symfony validation component Internationalization.
The point is instantiation of translator and validator.

composer.json

{
  "require" : {
    "twig/twig": "~1.0",
    "symfony/translation": "^3.1",
    "symfony/twig-bridge": "^3.1",
    "symfony/config": "^3.1",
    "symfony/form": "^3.1",
    "symfony/validator": "^3.1",
  },
}

php

use Symfony\Component\Translation\Translator;
use Symfony\Component\Translation\MessageSelector;
use Symfony\Component\Translation\Loader\XliffFileLoader
use Symfony\Component\Validator\Validation;
use Symfony\Component\Form\Extension\Validator\ValidatorExtension;

$translator = new Translator($locale, new MessageSelector());

$vendorDir = realpath(__DIR__.'/../vendor');
$vendorFormDir = $vendorDir.'/symfony/form';
$vendorValidatorDir = $vendorDir.'/symfony/validator';

$translator->addLoader('xlf', new XliffFileLoader());
// there are built-in translations for the core error messages
$translator->addResource('xlf', $vendorFormDir.'/Resources/translations/validators.en.xlf', 'en', 'validators');
$translator->addResource('xlf', $vendorValidatorDir.'/Resources/translations/validators.en.xlf', 'en', 'validators');
$translator->addResource('xlf', $vendorFormDir.'/Resources/translations/validators.ja.xlf', 'ja', 'validators');
$translator->addResource('xlf', $vendorValidatorDir.'/Resources/translations/validators.ja.xlf', 'ja', 'validators');

$validator = Validation::createValidatorBuilder()
  ->setTranslator($translator)
  ->setTranslationDomain('validators')
  ->getValidator();

$formFactory = Forms::createFormFactoryBuilder()
  ->addExtensions([new ValidatorExtension($validator)])
  ->getFormFactory();

$form = $formFactory->get()->createBuilder()
  ->add('email', TextType::class, array(
        'constraints' => array(
          new NotBlank(),
          new Length(array('min' => 4)),
          ),
        ))
  ->add('password', PasswordType::class, array(
        'constraints' => array(
          new NotBlank(),
          new Length(array('min' => 6)),
          ),
        ))
  ->getForm();

...

It is important that you specify validators as the domain, 4th argument of addResource method.
And it should correspond to the validator’s domain, which is specified with setTranslationDomain method.
You have to pass translator instance to the validator, too.

Please see the documentation if you want further explanation for domain. http://symfony.com/doc/current/components/translation/introduction.html#using-message-domains

That’s it!
You already have form with internationalized validation messages.

References:
http://symfony.com/doc/current/components/form/introduction.html#translation http://stackoverflow.com/questions/16063531/translation-in-validator-component