Do not use parameters.

anezi/locale-extension/1.0/config/packages/anezi_locale_extension.yaml

@@ -1,10 +1,8 @@
-parameters:
-    managed_locales: ['fr', 'en'] # Edit here for your needs.
-
 services:
     anezi_twig.locale_extension:
         class: Anezi\Locale\Twig\Extension\LocaleExtension
         public:    false
-        arguments: ["%managed_locales%"]
+        arguments:
+            $locales: ['fr', 'en'] # Edit here for your needs.
         tags:
             - { name: twig.extension }