I was stuck for a while.
You need to pass translator instance(also domain as needed) as an argument when you instantiate CsrfExtension.

php

use Symfony\Component\Form\Extension\Csrf\CsrfExtension;
use Symfony\Component\Form\Forms;

...

$csrfExtension = new CsrfExtension($csrfTokenManager, $translator, 'validators');

$formFactory = Forms::createFormFactoryBuilder()
  ->addExtensions([$csrfExtension])
  ->getFormFactory();

...

I wrote about internationalization of symfony validation component in older post,
however it seems each form extensions should be passed the Translator instance individually.

use Symfony\Component\Form\Extension\Csrf\CsrfExtension;
use Symfony\Component\Form\Extension\Validator\ValidatorExtension;
use Symfony\Component\Form\Forms;
use Symfony\Component\Security\Csrf\CsrfTokenManager;
use Symfony\Component\Translation\Translator;
use Symfony\Component\Validator\Validation;

...

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

// extensions
$validatorExtension = new ValidatorExtension($validator);
$csrfExtension = new CsrfExtension($csrfTokenManager, $translator, 'validators');

$formFactory = Forms::createFormFactoryBuilder()
  ->addExtensions([$csrfExtension, $validatorExtension])
  ->getFormFactory();