Index: Classes/DataHandling/SlugHelper.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- Classes/DataHandling/SlugHelper.php	(date 1568188868694)
+++ Classes/DataHandling/SlugHelper.php	(date 1568188868694)
@@ -250,7 +250,9 @@
         $this->applySlugConstraint($queryBuilder, $slug);
         $this->applyPageIdConstraint($queryBuilder, $pageId);
         $this->applyRecordConstraint($queryBuilder, $recordId);
-        $this->applyLanguageConstraint($queryBuilder, $languageId);
+        if ($languageId != -1) {
+            $this->applyLanguageConstraint($queryBuilder, [-1, $languageId]);
+        }
         $this->applyWorkspaceConstraint($queryBuilder);
         $statement = $queryBuilder->execute();
 
@@ -287,7 +289,9 @@
         $queryBuilder = $this->createPreparedQueryBuilder();
         $this->applySlugConstraint($queryBuilder, $slug);
         $this->applyRecordConstraint($queryBuilder, $recordId);
-        $this->applyLanguageConstraint($queryBuilder, $languageId);
+        if ($languageId != -1) {
+            $this->applyLanguageConstraint($queryBuilder, [-1, $languageId]);
+        }
         $this->applyWorkspaceConstraint($queryBuilder);
         $statement = $queryBuilder->execute();
 
@@ -431,9 +435,9 @@
 
     /**
      * @param QueryBuilder $queryBuilder
-     * @param int $languageId
+     * @param int|array $languageId
      */
-    protected function applyLanguageConstraint(QueryBuilder $queryBuilder, int $languageId)
+    protected function applyLanguageConstraint(QueryBuilder $queryBuilder, $languageId)
     {
         $languageFieldName = $GLOBALS['TCA'][$this->tableName]['ctrl']['languageField'] ?? null;
         if (!is_string($languageFieldName)) {
@@ -441,12 +445,21 @@
         }
 
         // Only check records of the given language
-        $queryBuilder->andWhere(
-            $queryBuilder->expr()->eq(
-                $languageFieldName,
-                $queryBuilder->createNamedParameter($languageId, \PDO::PARAM_INT)
-            )
-        );
+        if (is_array($languageId)) {
+            $queryBuilder->andWhere(
+                $queryBuilder->expr()->in(
+                    $languageFieldName,
+                    $queryBuilder->createNamedParameter($languageId, Connection::PARAM_INT_ARRAY)
+                )
+            );
+        } else {
+            $queryBuilder->andWhere(
+                $queryBuilder->expr()->eq(
+                    $languageFieldName,
+                    $queryBuilder->createNamedParameter($languageId, \PDO::PARAM_INT)
+                )
+            );
+        }
     }
 
     /**