Updated the LocalizationMiddleware middleware in the Infrastructure package to read explicit language requests.

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

@@ -5,11 +5,13 @@ import Localization
 
 /// Resolves the visitor's preferred language and records it on the request context.
 ///
-/// Placed ahead of the localized responders in the middleware chain, it reads the request's `Accept-Language` header, negotiates the best supported
-/// match (falling back to the default language), and stores it on the context's ``LocalizedRequestContext/language``.
+/// Placed ahead of the localized responders in the middleware chain, it reads the request's `lang` query parameter, its path, and its
+/// `Accept-Language` header, negotiates the best supported match (falling back to the default language), and stores it on the context's
+/// ``LocalizedRequestContext/language``.
 ///
-/// The request is otherwise passed through untouched — the URL and routing are not affected — so each page is served at its existing path and varies its
-/// content by header.
+/// The query parameter is the deliberate override a language switcher links to; failing that, a leading path segment naming a supported language
+/// pins it, so an unrouted path under a language's prefix — its not-found page — answers in that language. Values naming no supported language
+/// are ignored, leaving the header. The request is passed through untouched — the path and routing are unaffected.
 public struct LocalizationMiddleware<Context: LocalizedRequestContext> {
 
     // MARK: Properties
@@ -49,7 +51,13 @@ extension LocalizationMiddleware: RouterMiddleware {
     ) async throws -> Response {
         var context = context
 
+        // The deliberate query override first; failing that, the leading path segment, so a language's whole URL
+        // prefix — routed or not — answers in its language.
+        let requested = request.uri.queryParameters[.Parameter.language].map(String.init)
+            ?? request.uri.path.split(separator: "/").first.map(String.init)
+
         context.language = negotiate(
+            requested: requested,
             acceptLanguage: request.headers[.acceptLanguage]
         )
 
@@ -57,3 +65,13 @@ extension LocalizationMiddleware: RouterMiddleware {
     }
 
 }
+
+// MARK: - Constants
+
+private extension Substring {
+    /// A namespace for the query parameters the middleware reads.
+    enum Parameter {
+        /// The query parameter carrying an explicit language choice; the site's language switcher appends it to the current path.
+        static let language: Substring = "lang"
+    }
+}