Translation source language resets to English on import [#3409088]

Problem/Motivation

We have a bilingual setup with English as the site default language but German as the primary content language. Default content consists of nodes, terms, and menu links.

We noticed some confusion amongst our editors trying to edit untranslatable fields in the German translation, therefore we decided to swap the default content translations and make the German version the entities' default language.

Looking into the exported yml files, language settings are as intended:

default_langcode: de
content_translation_source: de

However, after import during site-install, all imported entities are set to English.

Translated entities or untranslated non-English entities - no difference.

I counter-tested with the default language set to German on site-install which results in imported content in German. This makes me wonder if the translation source is being assumed instead of being explicitely handled. Possible?

Steps to reproduce

Install a second language (but leave English as default)
Create a node in the secondary language
Export
Delete that node
Import

Issue fork default_content-3409088

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3409088-translation-source-language changes, plain diff MR !37
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