FormType.php

<?php

namespace AppBundle\Form\Type

use AppBundle\Form\DataTransformer\ContactPersonLanguageModelTransformer;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class AcmeType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            /** the actual collection used as choicetype now! **/
            ->add(
                'languages',
                ChoiceType::class,
                [
                    'choices' => array_flip(Intl::getLanguageBundle()->getLanguageNames()),
                    'preferred_choices' => ['de','en','el','fr','nl','dk','no','uk','cs','es','it','pt','pl','tr','ru','ja','zh'],
                    'choices_as_values' => true,
                    'placeholder' => 'please select a language',
                    'attr' => [
                        'style' => 'width: 100%;'
                    ],
                    'multiple' => true,
                    'expanded' => false
                ]
            );
        ;
        $builder->get('languages')
            ->addModelTransformer(new CollectionEntityPropertyToArrayModelTransformer('language'));
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(
            [
                'data_class' => 'AppBundle\Entity\AcmeEntity',
                'cascade_validation' => false,
                'attr' => ['novalidate' => 'novalidate']
            ]
        );
    }
}

ollectionEntityPropertyToArrayModelTransformer.php

<?php

namespace AppBundle\Form\DataTransformer;

use Doctrine\Common\Collections\Criteria;
use Symfony\Component\Form\DataTransformerInterface;
use Symfony\Component\PropertyAccess\PropertyAccess;

class CollectionEntityPropertyToArrayModelTransformer implements DataTransformerInterface
{
    /**
     * Name of the property of the entity
     *
     * @var string
     */
    protected $propertyName;

    /**
     *
     * @param string $propertyName
     */
    public function __construct($propertyName)
    {
        $this->propertyName = $propertyName;
    }

    /**
     * @var \Doctrine\ORM\PersistentCollection
     */
    private $persistentCollection;

    /**
     * Transforms an collection into an array with the currently propertyvalues
     *
     * @param \Doctrine\ORM\PersistentCollection|null $collection
     *
     * @return array
     */
    public function transform($collection)
    {
        $accessor = PropertyAccess::createPropertyAccessor();
        $array = [];
        $this->persistentCollection = $collection;
        /** @var \ArrayIterator $iterator */
        $iterator = $collection->getIterator();
        while ($iterator->valid()) {
            $array[] = $accessor->getValue($iterator->current(), $this->propertyName);
            $iterator->next();
        }

        return $array;
    }

    /**
     * Transforms an array to an collection
     *
     * @param  array $array
     *
     * @return \Doctrine\ORM\PersistentCollection
     */
    public function reverseTransform($array)
    {
        $accessor = PropertyAccess::createPropertyAccessor();

        $owner = $this->persistentCollection->getOwner();
        $targetEntity = $this->persistentCollection->getMapping()['targetEntity'];
        $mappedBy = $this->persistentCollection->getMapping()['mappedBy'];
        foreach ($array as $value) {
            $exists = Criteria::create()
                ->where(Criteria::expr()->eq($this->propertyName, $value));
            if ($this->persistentCollection->matching($exists)->count() <= 0) {
                $entity = new $targetEntity();
                /** Set owner */
                $accessor->setValue($entity, $mappedBy, $owner);
                /* Set property value */
                $accessor->setValue($entity, $this->propertyName, $value);
                $this->persistentCollection->add($entity);
            }
        }
        foreach ($this->persistentCollection as $entity) {
            if (!in_array($accessor->getValue($entity, $this->propertyName), $array)) {
                $this->persistentCollection->removeElement($entity);
            }
        }

        return $this->persistentCollection;
    }
}