Test that new fields are translatable when re-using existing storage but not translatable when added from the UI

+++ b/core/modules/field_ui/tests/src/Functional/ManageFieldsFunctionalTest.php
@@ -781,4 +781,23 @@ public function testNonExistentFieldUrls() {
+  public function testTranslationDefaults() {
+    $this->createField();
+    $field_storage = FieldStorageConfig::loadByName('node', 'field_' . $this->fieldNameInput);
+    $this->assertTrue($field_storage->isTranslatable(), 'Field storage settings were found.');
+
+    $field = FieldConfig::loadByName('node', $this->contentType, 'field_'. $this->fieldNameInput);
+    $this->assertFalse($field->isTranslatable(), 'Field storage settings were found.');
+
+    $this->addExistingField();
+    $field_storage = FieldStorageConfig::loadByName('node', 'field_' . $this->fieldNameInput);
+    $this->assertTrue($field_storage->isTranslatable(), 'Field storage settings were found.');
+
+    $field = FieldConfig::loadByName('node', $this->contentType, 'field_'. $this->fieldNameInput);
+    $this->assertFalse($field->isTranslatable(), 'Field storage settings were found.');

the message on the assertTrue/False is confusing, lets add a description for what we expect/why.

Conflicted with [##2717319] but I think it's still needed.

Yeah, old tests.

Also, conflicted again already in #3346682: Improve re-use an existing field user experience and I think this became kind of a duplicate as the refactoring there seems to have fixed this too.

I assume that was more an unintended side effect even though I didn't look at that other issue, so I doubt tests were added for this. Changing it to a task to add test coverage.