Issue #3516520 by larowlan, wim leers, penyaskito: LanguageItem lacks a schema...

Issue #3516520 by larowlan, wim leers, penyaskito: LanguageItem lacks a schema for its default values

(cherry picked from commit bc761e4a)

core/config/schema/core.data_types.schema.yml

@@ -940,6 +940,18 @@ field.value.timestamp:
       type: timestamp
       label: 'Value'
 
+field.value.language:
+  type: mapping
+  label: 'Language value'
+  mapping:
+    value:
+      # Not this doesn't make use of type: langcode because that doesn't allow null values.
+      type: string
+      label: 'Language value'
+      constraints:
+        Choice:
+          callback: 'Drupal\Core\TypedData\Plugin\DataType\LanguageReference::getAllValidLangcodes'
+
 # Text with a text format.
 text_format:
   type: mapping

core/tests/Drupal/KernelTests/Config/Schema/MappingTest.php

@@ -377,6 +377,7 @@ public static function providerMappingInterpretation(): \Generator {
         'field.value.decimal' => ['value'],
         'field.value.float' => ['value'],
         'field.value.timestamp' => ['value'],
+        'field.value.language' => ['value'],
         'field.value.comment' => [
           'status',
           'cid',