Use a pattern for fields validation

I need to validate the fields of user sent by an API. All works fine but the only one problem is that I don't like the solution is not an OOP solution. In the controller I call the service created by me:

$userValidatorService = $this->get(UserValidatorService::class);

And the service:

class UserValidatorService
{

/**
 * @var EntityManagerInterface
 */
private $entityManager;

/**
 * @var LoggerInterface;
 */
private $logger;

/**
 * @var TranslatorInterface
 */
private $translator;

/**
 * @var UserManagerInterface
 */
private $userManager;

/**
 * @var array
 */
private $locales;

/**
 * @var User
 */
private $user;

/**
 * @var array
 */
private $userFormData;

/**
 * UserValidatorService constructor.
 * @param EntityManagerInterface $entityManager
 * @param LoggerInterface $logger
 * @param TranslatorInterface $translator
 * @param UserManagerInterface $userManager
 */
public function __construct(
    EntityManagerInterface $entityManager,
    LoggerInterface $logger,
    TranslatorInterface $translator,
    UserManagerInterface $userManager
)
{
    $this->entityManager    = $entityManager;
    $this->logger           = $logger;
    $this->translator       = $translator;
    $this->userManager      = $userManager;
}
/**
 * @return array
 */
public function updateUser()
{
    $messages = [];

    if( '' !== $errorMessage = $this->validateEmail()){
        $messages['email'] = $errorMessage;
    }

    if( '' !== $errorMessage = $this->validateLocale()){
        $messages['locale'] = $errorMessage;
    // and all fields are validated like that
}
 /**
 * @return string
 */
private function validateEmail()
{
    if(isset($this->userFormData['email'])){
        if(!filter_var($this->userFormData['email'], FILTER_VALIDATE_EMAIL)){
            return $this->translator->trans('error.message');
        }
        return '';

    }
    return $this->translator->trans('error.message');
}

/**
 * @return string
 */
private function validateLocale()
{
    if(isset($this->userFormData['locale'])){
        if(!in_array($this->userFormData['locale'], $this->locales)){
            return $this->translator->trans('error.message');
        }
        return '';
    }
    return $this->translator->trans('error.message');
}

The question exists as a pattern to use to avoid this duplication and to do more OOP (I repeat that this code works just fine). I want to do that by myself without using Symfony validators.