How to translate Symfony 3 forms?

Translating content in twig templates seems easy and straightforward: I run bin/console translation:update to populate translation files. The issue is that it seems to ignore forms.

After creating entities, repositories and forms/types, I use {{ form_row(form.field) }} in twig templates to draw the form elements.

Is there a well-established practice on how to customize and translate form labels, placeholders and errors messages?

Must I use form_label and form_widget instead of form_row to customize labels?

You could create for example a file named forms.es.yml here, you can put you traductions in spanish, and in your forms you can chain it like this:

//LoginType.php
public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        ->add('email',
            EmailType::class,
            [
                'translation_domain' => 'forms', //It's translate the label
                'constraints' => [
                    new Email([
                        'message' => 'email'
                    ])
                ]
            ]
        )
        ->add('password',
            RepeatedType::class,
            [
                'type' => PasswordType::class,
                'invalid_message' => 'cliente.password_not_equal',
                'first_options' => ['label' => 'cliente.password'],
                'second_options' => ['label' => 'cliente.repeat_password'],
                'translation_domain' => 'forms', //Here is again
                'constraints' => [
                    new NotBlank([
                        'message' => 'not_blank'
                    ])
                ]
            ]
        )
        ->add('current_uri', HiddenType::class);
}

It works since symfony 2.

Another way is in your twig, only print the widget and translate the label:

//index.html.twig
<label> 
   {{'form.email'|trans({})}}
   {{ form_widget('form.email') }} //It only prints the input tag
</label>

Is an easy way, but is less reusable.