Translations of the parent entity are lost when creating a child entity with an Inline entity form

Debugging the issue, I've found that, if a child entity has been created in the inline entity form, in the call EntityTranslationDefaultHandler::updateTranslations() from entity_translation_field_attach_update(), $langcode is set to the edited translation language and $this->isTranslating() is false, so EntityTranslationDefaultHandler::setOriginalLanguage() is called. This remove the original language translation from the handler's own translations data structure. Which later replace the translations stored in the database.

However, when a child entity has not been created in the inline entity form $langcode is the original translation's language. So the call to EntityTranslationDefaultHandler::setOriginalLanguage() does not remove the original translation.

Next step is to identify what causes the $langcode to have the wrong value.

When a child entity has been created from the edit form for the translation of the parent, in EntityTranslationDefaultHandler::updateFormLanguage, $form_state['values']['language'] is set to the edited translation language. If a child entity has not been created, $form_state['values']['language'] is set to the original translation language.

I tried applying the patch #1865244: Allow multiple translation handlers on the same form but it does not help (actually, that's the first thing Itried and I've continued working with the patch applied).

When the child entity creation form embedded in the edit form for the translation of the parent entity is submitted,
EntityTranslationDefaultHandler::entityFormLanguageWidget() is called twice on the same $form. The first time for the parent entity, the second time for the child entity. Off course, the second time the method is called, the child entity is new and is not a translation.

The first call to EntityTranslationDefaultHandler::entityFormLanguageWidget() creates $form['language'] as a select element whose default value is the language of the original translation of the parent entity. The second call to EntityTranslationDefaultHandler::entityFormLanguageWidget() resets $form['language'] to a new select element whose default value is the language of the child entity.

The easiest fix is to not enabled translation of the child entity type.

The issue happens when the translation is enabled for the child entity type.

I have multiple translation handler on the same form. They both add a language selector to the form, with the same key. So the second handler ends up overriding the first handler's language selector.