Translatable strings shouldn't be used as array keys. They can easily be changed from the UI, causing things to break.
In Drupal\Core\Entity\EntityTypeRepository::getEntityTypeLabels() line 52, we have the following code:
$options[(string) $definition->getGroupLabel()][$entity_type_id] = $definition->getLabel();
$definition->getGroupLabel() returns a translatable string, and shouldn't be used as an array key.
We could probably just use $definition->getGroup() as array key instead.
The change in core would be fairly simple, but there are a lot of contrib modules depend on this, in the sense that they use code similar to this:
$labels = \Drupal::service('entity_type.repository')->getEntityTypeLabels(TRUE);
$label = $labels[(string) t('Content', [], ['context' => 'Entity type group'])];
Needs work
10.1 ✨
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The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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As a bug this will need a test case.