Configuration translated to default-language make configuration stale

Problem/Motivation

I found this in a legacy project so not sure how this happened but it's really ugly to debug.

The situation:

$ drush cget system.site
name: 'Foobar GmbH'
langcode: de
default_langcode: de
...

sshd-3657828677-m3whw:/var/www/html/docroot$ drush cget system.site --include-overridden
name: Drupal
langcode: de
default_langcode: de
...

As we see (also in config/sync), we have conflicting config and default-language-overridden config.
What now happens:
* User sees one site name (the overridden)
* Any attempt to change the site name only changes the un-overridden value

This makes some config stale, thus the site unusable, so prio is major or critical.

How to reproduce (untested)

I guess this is an instance of the admin/config/regional/language warning "It is not recommended to change the default language on a working site.". (Which is a big understatement here.)

* Install an english site with site name "Drupal"
* Enable german language
* Translate german site name to "Foobar"
* Change default language to german

Workaround

TODO: Merge and delete exported default-language-overrides.
(Just copying won't help as config may be overridden partly.)

Untested:

drush cex -y # if not done already
cexdir=../config-sync

lang=$(drush cget system.site default_langcode --format=string)

# Merge all default-language-translations and delete them
for filename in $cexdir/language/$lang/*.yml; do
  basename=${filename##*/}
  configname=${basename%.yml}
  # drush cset configname "" - --format=yaml <$filename # wont work...
  drupal yaml:merge $cexdir/$basename $cexdir/$basename $filename
  # Restore 4->2 spaces indent.
  sed 's/^\( \+\)\1\1\1/\1\1/' -i $cexdir/$basename
  rm $filename
done

# Give all untranslated config default language
langyml=$(mktemp)
echo "langcode: $lang" >$langyml
for filename in $cexdir/*.yml; do
  drupal yaml:merge $filename $filename $langyml
  # Restore 4->2 spaces indent.
  sed 's/^\( \+\)\1\1\1/\1\1/' -i $filename
done

# Optionally select which of the changes you want.

drush cim

Proposed resolution

a) Ensure integrity
Merge and drop any default-language-overrides:
* When saving config
* When changing default language
* When importing config
* (any other?)
...to ensure there are no default-language-overrides.

b) Alternatively, allow to translate to default language.

c) Alternatively, store all un-overridden config in neutral ("und") language
At first thought, this looks like the most elegant solution to me.
It draws a clear line between config and its translation, making eg distribution update workflows simpler of even feasible at all.
TODO: Consider if this drops legacy from the D7 way of thinking, or if this resolution has other drawbacks.

Code to check

* \Drupal\language\LanguageListBuilder::submitForm
* \Drupal\language\EventSubscriber\ConfigSubscriber::onConfigSave

Remaining tasks

* Find out how to reproduce
* Make it a failing test
* Code the above resolution

User interface changes

None

API changes

None

Data model changes

None