d6_url_alias_language should accept language as a scalar

Follow-up to #2226455: Migrated URL aliases not working until nodes are re-saved

+++ b/core/modules/path/migration_templates/d6_url_alias.yml
@@ -17,6 +17,9 @@ process:
+  langcode:
+    plugin: d6_url_alias_language
+    source:
+      - language

+++ b/core/modules/path/src/Plugin/migrate/process/d6/UrlAliasLanguage.php
@@ -0,0 +1,33 @@
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    $language = reset($value);

Oops! anavarre picked this up, there's no reason for the source value to be an array... we should be able to specify the YAML as

source: language

but the reset() complains.