feat: public API, SEO and existing surfaces

features/site_languages.qa.md

@@ -42,7 +42,7 @@ Mock /v1 per case. Do not freeze the system date.
 - Row menu: Make default, Disable, Remove.
 - Disabled language in the picker: "Disabled — re-enable instead".
 - Version counts are visible to every signed-in reader; the exception is recorded in docs/.
-- Social locale rule 3 (Site.Locale language part) is deferred to slice 6.
+- Social locale rule 3 (Site.Locale language part) is the `derived` state, covered by features/localization_public_api_and_surfaces.qa.md, item 7.
 
 ## Fix round additions