diff -u b/core/modules/content_translation/migrations/d6_node_master.yml b/core/modules/content_translation/migrations/d6_node_master.yml
--- b/core/modules/content_translation/migrations/d6_node_master.yml
+++ b/core/modules/content_translation/migrations/d6_node_master.yml
@@ -1,3 +1,4 @@
+# Migrates all revisions and all revision translations.
 id: d6_node_master
 label: Node Master
 audit: true
diff -u b/core/modules/content_translation/migrations/d7_node_master.yml b/core/modules/content_translation/migrations/d7_node_master.yml
--- b/core/modules/content_translation/migrations/d7_node_master.yml
+++ b/core/modules/content_translation/migrations/d7_node_master.yml
@@ -1,3 +1,4 @@
+# Migrates all revisions and all revision translations.
 id: d7_node_master
 label: Nodes
 audit: true
diff -u b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentMaster.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentMaster.php
--- b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentMaster.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentMaster.php
@@ -39,6 +39,9 @@
 
   /**
    * {@inheritdoc}
+   *
+   * @todo: Use EntityFieldDefinitionTrait instead.
+   *   See https://www.drupal.org/project/drupal/issues/2937782.
    */
   protected static function getEntityTypeId($plugin_id) {
     // Remove entity_revision:
diff -u b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
--- b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -971,6 +971,7 @@
    * {@inheritdoc}
    */
   public function getHighestId() {
+    // Ensure that at least one Id is an integer.
     if (array_search('integer', array_column($this->migration->getDestinationPlugin()->getIds(), 'type')) === FALSE) {
       throw new \LogicException('Cannot determine the highest migrated ID without an integer ID column');
     };
diff -u b/core/modules/migrate_drupal/src/MigrationConfigurationTrait.php b/core/modules/migrate_drupal/src/MigrationConfigurationTrait.php
--- b/core/modules/migrate_drupal/src/MigrationConfigurationTrait.php
+++ b/core/modules/migrate_drupal/src/MigrationConfigurationTrait.php
@@ -103,12 +103,20 @@
     /** @var \Drupal\migrate\Plugin\Migration[] $all_migrations */
     $all_migrations = $plugin_manager->createInstancesByTag($version_tag);
 
+    // If this source database is multilingual then we must run only the
+    // dN_node_master migration and not any other dN_node* migration.
+    // Conversely, if the source database is not multilingual then we want to
+    // run all dN_node* migrations except dN_node_master. Here we unset the
+    // migrations we don't want to run.
     if ($this->isMultilingualSource($database_state_key, $drupal_version)) {
       $patterns = '/(d' . $drupal_version . '_node:)|(d' . $drupal_version . '_node_translation:)|(d' . $drupal_version . '_node_revision:)/';
-      foreach ($all_migrations as $key => $migrations) {
-        if (preg_match($patterns, $key)) {
-          unset($all_migrations[$key]);
-        }
+    }
+    else {
+      $patterns = '/(d' . $drupal_version . '_node_master:)/';
+    }
+    foreach ($all_migrations as $key => $migrations) {
+      if (preg_match($patterns, $key)) {
+        unset($all_migrations[$key]);
       }
     }
 
@@ -221,7 +229,7 @@
   }
 
   /**
-   * Determines if the source database uses i18n module.
+   * Determines if the i18n module is installed in the source database.
    *
    * @return bool
    *   The version if this is a source database with i18n, FALSE otherwise.
@@ -233,7 +241,7 @@
     $connection = $this->getConnection($database_state['database']);
     if ($connection) {
       $status = $connection->query("SELECT status FROM {system} WHERE name = :name  and type = :type", [
-        ':name' => 'system',
+        ':name' => 'i18n',
         ':type' => 'module',
       ])->fetchField();
     }
diff -u b/core/modules/migrate_drupal_ui/src/Form/IdConflictForm.php b/core/modules/migrate_drupal_ui/src/Form/IdConflictForm.php
--- b/core/modules/migrate_drupal_ui/src/Form/IdConflictForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/IdConflictForm.php
@@ -74,7 +74,11 @@
 
     $results = (new IdAuditor())->auditMultiple($migrations);
 
-    // Get the audit results for node revisions for the node_master migrations.
+    // If dN_node_master migration is being used get the audit results for node
+    // revisions manually. We need to do this manually because the node master
+    // map has the ids for both nodes and revisions and Sql::getHighestId()
+    // only returns the highest migrated ID of the destination entity type,
+    // which for node_master is node.
     $migration_plugin_manager = \Drupal::service('plugin.manager.migration');
     foreach ($migrations as $migration) {
       $migration_id = $migration->getPluginId();
diff -u b/core/modules/node/src/Plugin/migrate/source/d6/NodeMaster.php b/core/modules/node/src/Plugin/migrate/source/d6/NodeMaster.php
--- b/core/modules/node/src/Plugin/migrate/source/d6/NodeMaster.php
+++ b/core/modules/node/src/Plugin/migrate/source/d6/NodeMaster.php
@@ -3,7 +3,7 @@
 namespace Drupal\node\Plugin\migrate\source\d6;
 
 /**
- * Master node source.
+ * Gets all node revisions from the source, including translation revisions.
  *
  * @MigrateSource(
  *   id = "d6_node_master",
diff -u b/core/modules/node/src/Plugin/migrate/source/d7/NodeMaster.php b/core/modules/node/src/Plugin/migrate/source/d7/NodeMaster.php
--- b/core/modules/node/src/Plugin/migrate/source/d7/NodeMaster.php
+++ b/core/modules/node/src/Plugin/migrate/source/d7/NodeMaster.php
@@ -5,7 +5,7 @@
 use Drupal\Core\Database\Query\SelectInterface;
 
 /**
- * Master node source.
+ * Gets all node revisions from the source, including translation revisions.
  *
  * @MigrateSource(
  *   id = "d7_node_master",
@@ -52,6 +52,5 @@
    * {@inheritdoc}
    */
-  protected function handleTranslations(SelectInterface $query) {
-  }
+  protected function handleTranslations(SelectInterface $query) {}
 
 }