->add(
    'base_language',
    ChoiceType::class,
    [
        'required' => false,
        'placeholder' => false,
        'multiple' => false,
        'expanded' => false,
        'choice_loader' => new CallbackChoiceLoader(function () use ($contentLanguages) {
            return $this->loadLanguages(
                function (Language $language) use ($contentLanguages) {
                    return $language->enabled && in_array($language->languageCode, $contentLanguages, true);
                }
            );
        }),
        'choice_value' => 'languageCode',
        'choice_label' => 'name',
    ]
);

 $('#add-translation_base_language').prepend("<option value='' selected='selected'></option>");