Allow selecting default posting language instead of auto-detect (#7828)

* Allow selecting default posting language instead of auto-detect

* Enable default language setting in credentials API

* Fix form saving

app/serializers/rest/credential_account_serializer.rb

@@ -9,6 +9,7 @@ class REST::CredentialAccountSerializer < REST::AccountSerializer
     {
       privacy: user.setting_default_privacy,
       sensitive: user.setting_default_sensitive,
+      language: user.setting_default_language,
       note: object.note,
       fields: object.fields.map(&:to_h),
     }