Enforce source language set wrong source field in content_translation

The following patch solves the problem for us:

diff --git a/sources/content/src/Plugin/tmgmt/Source/ContentEntitySource.php b/sources/content/src/Plugin/tmgmt/Source/ContentEntitySource.php
index b101d7b2..099d1b78 100644
--- a/sources/content/src/Plugin/tmgmt/Source/ContentEntitySource.php
+++ b/sources/content/src/Plugin/tmgmt/Source/ContentEntitySource.php
@@ -498,7 +498,7 @@ class ContentEntitySource extends SourcePluginBase implements SourcePreviewInter
     $translation = $entity->getTranslation($target_langcode);
     $manager = \Drupal::service('content_translation.manager');
     if ($manager->isEnabled($translation->getEntityTypeId(), $translation->bundle())) {
-      $manager->getTranslationMetadata($translation)->setSource($entity->language()->getId());
+      $manager->getTranslationMetadata($translation)->setSource($item->getJob()->getSourceLangcode());
     }
 
     foreach (Element::children($data) as $field_name) {