InvalidArgumentException: Cannot translate if field references a translation not in the source language

Summary

An English node cannot be translated to German if a field using IEF references Finnish content (and that reference has no English translation).

Reproducible with a fresh site on https://simplytest.me:

• Drupal Core 8.3.7
• Inline Entity Form 8.x-1.0-beta1

Preparation:

Site:

1. Install standard site with English
2. Install Inline Entity Form module at /admin/modules
3. Install Content Translation module at /admin/modules (enables Language module)
4. Add German and Finnish at /admin/config/regional/language/add
5. Change error display: All messages, with backtrace information at /admin/config/development/logging

Content type:

1. Enable translation for Article and "Show language selector on create and edit pages" at /admin/structure/types/manage/article
2. Add a new field to Reference to Content, called "Card list" at /admin/structure/types/manage/article/fields/add-field and reference Articles
3. Manage the form display at /admin/structure/types/manage/article/form-display and select "Inline Entity Form - Complex" for "Card list", then "Allow users to add existing nodes"

Content:

1. Create "Finnish article to reference" at /node/add/article and save as Finnish
2. Create "English article that references a Finnish one" at /node/add/article, add the existing Finnish node and save as English

To reproduce:

1. Open "English article that references a Finnish one"
2. Click the Translate tab
3. Click "Add" next to German

Expected result:

Form opens to edit and save translation

Actual result:

The website encountered an unexpected error. Please try again later.

InvalidArgumentException: Invalid translation language (en) specified. in Drupal\Core\Entity\ContentEntityBase->getTranslation() (line 783 of core/lib/Drupal/Core/Entity/ContentEntityBase.php).
Drupal\inline_entity_form\TranslationHelper::prepareEntity(Object, Object) (Line: 338)
Drupal\inline_entity_form\Plugin\Field\FieldWidget\InlineEntityFormBase->prepareFormState(Object, Object, 1) (Line: 215)
Drupal\inline_entity_form\Plugin\Field\FieldWidget\InlineEntityFormComplex->formElement(Object, 0, Array, Array, Object) (Line: 322)
Drupal\Core\Field\WidgetBase->formSingleElement(Object, 0, Array, Array, Object) (Line: 85)
Drupal\Core\Field\WidgetBase->form(Object, Array, Object, NULL) (Line: 532)
Drupal\inline_entity_form\Plugin\Field\FieldWidget\InlineEntityFormBase->form(Object, Array, Object) (Line: 168)
Drupal\Core\Entity\Entity\EntityFormDisplay->buildForm(Object, Array, Object) (Line: 122)
Drupal\Core\Entity\ContentEntityForm->form(Array, Object) (Line: 98)
Drupal\node\NodeForm->form(Array, Object) (Line: 115)
Drupal\Core\Entity\EntityForm->buildForm(Array, Object)
call_user_func_array(Array, Array) (Line: 514)
Drupal\Core\Form\FormBuilder->retrieveForm('node_article_form', Object) (Line: 271)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 48)
Drupal\Core\Entity\EntityFormBuilder->getForm(Object, 'default', Array) (Line: 350)
Drupal\content_translation\Controller\ContentTranslationController->add(Object, Object, Object, 'node')
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 574)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array) (Line: 144)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 64)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 656)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
02:28:25 drupal 8.3.7 ✘

The backtrace points the finger at inline_entity_form.

Additional notes:

1. Change the Card list to use Autocomplete instead of IEF at /admin/structure/types/manage/article/form-display
2. You can now translate to German.