<?php
$input = array(
   'one'    => 'two',
   'three'  => 'four',
);

// transforms to...
$transformed = array(
    array('internal_name' => 'two', 'output_name' => 'one'),
    array('internal_name' => 'four', 'output_name' => 'three'),
);

$builder->addEventListener(FormEvents::PRE_SET_DATA, $listener, 100);

<?php

namespace MyNamespaceForm;

use Symfony\Component\Form\DataTransformerInterface;
use Symfony\Component\Form\Exception\TransformationFailedException;

class TranslationTransformer implements DataTransformerInterface
{
    public function transform($translations)
    {
        if ($translations === null) {
            return [];
        }

        $formData = [];
        foreach ($translations as $id => $value) {
          $formData[] = ["id" => $id, "value" => $value];
        }

        return $formData;
    }

    public function reverseTransform($formData)
    {
        $data = [];
        foreach ($formData as $line) {
          if (isset($data[$line['id']])) {
            $failure = new TransformationFailedException("Duplicated key");
            $failure->setInvalidMessage("workspace.translation.duplicate_key");

            throw $failure;
          }
          $data[$line['id']] = $line['value'];
        }

        if (count($data) === 0) {
          return null;
        }

        return $data;
    }
}