Allow entity types to override EntityType::getLowercaseLabel()

Add optional "lowercase_label" key to ConfigEntityType/ContentEntityType annotation?

That sounds for me like the best, because simplest, solution. Are you sure we need the additional parameter on the label() method?

Given that a new entity key also allows you to not break any kind of API, it should be pretty okay for 8.1

Makes sense, but to fix #2507235: EntityType::getLowercaseLabel() breaks translation, we need to add it to all entity types and we need them all to have a 'entity-type-label-lowercase' or similar context so german can translate "content" to uppercase "Inhalt". Maybe we should even enforce that context?

+++ b/core/tests/Drupal/Tests/Core/Entity/EntityTypeTest.php
@@ -299,6 +299,22 @@ public function testGetLabel() {
 
   /**
+   * @covers ::getLowercaseLabel
+   */
+  public function testGetLowercaseLabel() {
+    // The lowercase label should be used if defined.
+    $entity_type = $this->setUpEntityType(['label' => 'Thing ABC']);
+    $this->assertSame('thing abc', $entity_type = $entity_type->getLowercaseLabel());
+    $entity_type = $this->setUpEntityType(['label' => 'Thing ABC', 'lowercase_label' => 'thing ABC']);
+    $this->assertSame('thing ABC', $entity_type = $entity_type->getLowercaseLabel());
+
+    // With markup object.
+    $translatable_lowercase_label = new TranslatableMarkup($this->randomMachineName());
+    $entity_type = $this->setUpEntityType(array('lowercase_label' => $translatable_lowercase_label));
+    $this->assertSame($translatable_lowercase_label, $entity_type->getLabel());
+  }

Ideally you would use two test methods for the two cases.

Patch looks fine. It would be nice to figure out how will this work if/once #1850080: Entity type labels lack plurality, cannot generate UI text based on label if plural is needed is added too. You don't need to resolve it here, just saying that is also in the works.

+++ b/core/modules/rdf/src/Entity/RdfMapping.php
@@ -17,6 +17,7 @@
+ *   lowercase_label = @Translation("RDF mapping"),

I agree it would be important to have a "Lowercase label" context. It needs to be in the annotation for potx to parse it properly, so not much of a chance to automate that at another place.

Should we close this in favor of #2507235: EntityType::getLowercaseLabel() breaks translation BTW?

I guess that makes sense, we can re-open if the direction will change there but I don't think so.