Updated the Localized HTML Collection response in the Infrastructure package to make Vary: Accept-Language opt-in for localized responses.

Packages/Infrastructure/Sources/Public/Middlewares/NotFoundMiddleware.swift

@@ -7,6 +7,9 @@ import Hummingbird
 /// Placed ahead of `FileMiddleware` in the middleware chain, it catches the `.notFound` error that bubbles up when no file exists for the requested
 /// path and responds with the rendered error page and a `404 Not Found` status. The page is served in the language stored on the context by
 /// ``LocalizationMiddleware``, falling back to the default language.
+///
+/// Its responses declare `Vary: Accept-Language`: where the page routes pin their language by URL, this is the one responder that negotiates.
+/// An unrouted path names no edition, so no canonical URL contradicts the header.
 public struct NotFoundMiddleware<Context: LocalizedRequestContext> {
 
     // MARK: Properties
@@ -27,6 +30,7 @@ public struct NotFoundMiddleware<Context: LocalizedRequestContext> {
         self.responses = .init(
             bundle: bundle,
             status: .notFound,
+            variesOnAcceptLanguage: true,
             document: document
         )
     }