Localization support for the Website service (#10)

This PR contains the work done to introduce server-side localization support to the **Website** service so the landing and error pages are served in the visitor's negotiated language, backed by a new reusable Localization package.

To provide further details about the work:

* Created the _Localization_ package — a bundle-bound `Localize` method and a `LanguageList` type.
* Language negotiation — `NegotiateLanguage` method picks the best supported language from the request's _Accept-Language_ header (falling back to the default); the `LocalizationMiddleware` middleware resolves it per request and stores it on a new `LocalizedRequestContext` / `WebsiteRequestContext` context.
* Localized responses — `LocalizedHTMLCollectionResponse` pre-renders each page once per language and caches the bytes (with `Content-Language` + `Vary: Accept-Language`), reused by the  `RootController` controller and `NotFoundMiddleware` middleware.
* The `CachedHTMLResponse` response gained custom-header support.
* Localized pages — the `IndexPage` and `ErrorPage` pages now resolve their strings via `Localize` method;
* Added `Localizable.xcstrings` catalogs.
* Wired the `LocalizationMiddleware` middlewaer into the router.

Reviewed-on: rock-n-code/loud-amsterdam#10
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Sources/Library/Internal/Methods/NegotiateLanguage.swift

@@ -0,0 +1,85 @@
+import Foundation
+import Localization
+
+/// Negotiates the best supported language for a request from its `Accept-Language` header.
+///
+/// Bound to the module's catalog languages via ``LanguageList``, an instance is invoked like a
+/// function — through ``callAsFunction(forAcceptLanguage:)`` — to resolve a header value to a
+/// supported language identifier, falling back to the default language.
+struct NegotiateLanguage {
+
+    // MARK: Properties
+
+    /// The supported languages and default language, derived from the module's String Catalog.
+    private let list: LanguageList
+
+    // MARK: Initializers
+
+    /// Creates a language negotiator backed by the module's String Catalog.
+    init() {
+        self.list = .init(bundle: .module)
+    }
+
+    // MARK: Methods
+
+    /// Picks the best supported language for the given `Accept-Language` header value.
+    ///
+    /// Invoked by calling the instance directly, for example `negotiate(forAcceptLanguage: header)`.
+    /// The header is split into its language tags (dropping any `q` weights), then matched against
+    /// the supported languages with `Bundle.preferredLocalizations(from:forPreferences:)`. When the
+    /// header is absent or matches nothing, the default language is returned.
+    /// - Parameter acceptLanguage: the raw `Accept-Language` header value, if any.
+    /// - Returns: the identifier of the supported language to serve.
+    func callAsFunction(
+        forAcceptLanguage acceptLanguage: String?
+    ) -> String {
+        guard let acceptLanguage else {
+            return list.default
+        }
+
+        let tags = tags(from: acceptLanguage)
+
+        guard !tags.isEmpty else {
+            return list.default
+        }
+
+        let preferred = Bundle.preferredLocalizations(
+            from: list.all,
+            forPreferences: tags
+        )
+
+        return preferred.first
+            ?? list.default
+    }
+
+}
+
+// MARK: - Helpers
+
+private extension NegotiateLanguage {
+
+    // MARK: Methods
+
+    /// Extracts the ordered language tags from an `Accept-Language` header value.
+    ///
+    /// Each comma-separated entry is reduced to its language tag by dropping the `;q=` weight, and
+    /// blank entries are removed. The original order is kept, which mirrors descending preference
+    /// closely enough for `preferredLocalizations(from:forPreferences:)` to resolve correctly.
+    /// - Parameter acceptLanguage: the raw `Accept-Language` header value.
+    /// - Returns: the ordered, weight-stripped language tags.
+    func tags(
+        from acceptLanguage: String
+    ) -> [String] {
+        acceptLanguage
+            .split(separator: ",")
+            .map { entry in
+                entry
+                    .split(separator: ";")
+                    .first
+                    .map(String.init)?
+                    .trimmingCharacters(in: .whitespaces) ?? ""
+            }
+            .filter { !$0.isEmpty }
+    }
+
+}

Services/Website/Sources/Library/Internal/Pages/ErrorPage.swift

@@ -1,14 +1,39 @@
 import Elementary
+import Foundation
+import Localization
 
-/// The HTML page rendered for a not-found response.
+/// The HTML page rendered for a not-found response, with its text localized to a given locale.
 struct ErrorPage: HTMLDocument, Sendable {
 
+    // MARK: Properties
+
+    /// The locale the page content is localized to.
+    private let locale: Locale
+
+    /// Resolves the page's text from the bundled String Catalog for the page's ``locale``.
+    private let localize: Localize
+
+    // MARK: Initializers
+
+    /// Creates a not-found page localized to the given locale.
+    /// - Parameter locale: the locale the page content is localized to.
+    init(
+        locale: Locale
+    ) {
+        self.locale = locale
+        self.localize = .init(bundle: .module)
+    }
+
     // MARK: Document
 
-    /// The page's content.
+    /// The page's content: a localized heading and explanatory message.
     var body: some HTML {
-        h1 { "Page Not Found" }
-        p { "Sorry, but the page you were trying to view does not exist." }
+        h1 {
+            localize("error.heading", locale: locale)
+        }
+        p {
+            localize("error.message", locale: locale)
+        }
     }
 
     /// The metadata and stylesheet link placed in the document head.
@@ -24,10 +49,15 @@ struct ErrorPage: HTMLDocument, Sendable {
         )
     }
 
-    /// The document language.
-    var lang: String { "en" }
+    /// The document language, derived from the page's locale and falling back to the default language.
+    var lang: String {
+        locale.language.languageCode?.identifier
+            ?? LanguageList(bundle: .module).default
+    }
 
-    /// The document title.
-    var title: String { "Page Not Found" }
+    /// The localized document title.
+    var title: String {
+        localize("error.title", locale: locale)
+    }
 
 }

Services/Website/Sources/Library/Internal/Pages/IndexPage.swift

@@ -1,13 +1,36 @@
 import Elementary
+import Foundation
+import Localization
 
-/// The website's landing page.
+/// The website's landing page, with its text localized to a given locale.
 struct IndexPage: HTMLDocument, Sendable {
 
+    // MARK: Properties
+
+    /// The locale the page content is localized to.
+    private let locale: Locale
+
+    /// Resolves the page's text from the bundled String Catalog for the page's ``locale``.
+    private let localize: Localize
+
+    // MARK: Initializers
+
+    /// Creates a landing page localized to the given locale.
+    /// - Parameter locale: the locale the page content is localized to.
+    init(
+        locale: Locale
+    ) {
+        self.locale = locale
+        self.localize = .init(bundle: .module)
+    }
+
     // MARK: Document
 
-    /// The page's content.
+    /// The page's content: a localized greeting followed by the app script.
     var body: some HTML {
-        p { "Hello world! This is HTML5 Boilerplate." }
+        p {
+            localize("index.greeting", locale: locale)
+        }
         script(.src("/js/app.js")) {}
     }
 
@@ -52,14 +75,15 @@ struct IndexPage: HTMLDocument, Sendable {
         )
     }
 
-    /// The document language.
+    /// The document language, derived from the page's locale and falling back to the default language.
     var lang: String {
-        "en"
+        locale.language.languageCode?.identifier
+            ?? LanguageList(bundle: .module).default
     }
 
-    /// The document title.
+    /// The localized document title.
     var title: String {
-        "Index page"
+        localize("index.title", locale: locale)
     }
 
 }

Services/Website/Sources/Library/Internal/Responses/CachedHTMLResponse.swift

@@ -1,12 +1,16 @@
 import Elementary
+import HTTPTypes
 import Hummingbird
 import NIOCore
 
 /// A pre-rendered HTTP response for a fully static HTML page.
 ///
 /// The document is rendered to bytes once, at initialization, and every ``response()`` reuses those
-/// bytes instead of re-rendering. This suits pages whose markup never changes between requests — the
-/// landing page and the not-found page — avoiding a per-request Elementary render on hot paths.
+/// bytes — along with a fixed status and precomputed headers — instead of re-rendering. This suits
+/// pages whose markup never changes between requests, such as the landing page and the not-found
+/// page, avoiding a per-request Elementary render on hot paths.
+///
+/// ``LocalizedHTMLResponses`` builds on this type, caching one instance per supported language.
 ///
 /// The body is written as an unsized stream (no `Content-Length`), mirroring `HTMLResponse`, so the
 /// response-compression middleware downstream treats it exactly as it would a freshly rendered page.
@@ -14,23 +18,37 @@ struct CachedHTMLResponse: Sendable {
 
     // MARK: Properties
 
-    /// The status applied to every response.
-    private let status: HTTPResponse.Status
     /// The page rendered to bytes once.
     private let buffer: ByteBuffer
+    /// The headers applied to every response, precomputed once.
+    private let headers: HTTPFields
+    /// The status applied to every response.
+    private let status: HTTPResponse.Status
 
     // MARK: Initializers
 
     /// Renders the given document to bytes once.
     /// - Parameters:
     ///   - status: the status applied to every response. Defaults to `.ok`.
+    ///   - additionalHeaders: extra headers merged onto every response, alongside the content type.
+    ///     Used to carry per-language signals such as `Content-Language` and `Vary`.
     ///   - document: the static HTML document to render and cache.
     init(
         status: HTTPResponse.Status = .ok,
-        _ document: some HTMLDocument
+        additionalHeaders: HTTPFields = [:],
+        document: some HTMLDocument
     ) {
+        var headers: HTTPFields = [
+            .contentType: "text/html; charset=utf-8"
+        ]
+
+        for field in additionalHeaders {
+            headers[field.name] = field.value
+        }
+
         self.status = status
-        self.buffer = ByteBuffer(string: document.render())
+        self.headers = headers
+        self.buffer = .init(string: document.render())
     }
 
     // MARK: Methods
@@ -43,7 +61,7 @@ struct CachedHTMLResponse: Sendable {
     func response() -> Response {
         Response(
             status: status,
-            headers: [.contentType: "text/html; charset=utf-8"],
+            headers: headers,
             body: .init { [buffer] writer in
                 try await writer.write(buffer)
                 try await writer.finish(nil)

Services/Website/Sources/Library/Internal/Responses/LocalizedHTMLCollectionResponse.swift

@@ -0,0 +1,65 @@
+import Elementary
+import Foundation
+import HTTPTypes
+import Hummingbird
+import Localization
+
+/// A per-language collection of pre-rendered HTML responses.
+///
+/// At initialization it renders the document once for each language the bundle's ``LanguageList``
+/// reports and caches the bytes, mirroring ``CachedHTMLResponse``'s render-once model but keyed by
+/// language. Each cached response carries a `Content-Language` header and `Vary: Accept-Language`, so
+/// shared caches key on the negotiated language instead of serving one language to everyone.
+struct LocalizedHTMLCollectionResponse: Sendable {
+
+    // MARK: Properties
+
+    /// The supported languages and default language, derived from the module's String Catalog.
+    private let list: LanguageList
+
+    /// The pre-rendered responses, keyed by language identifier.
+    private let responses: [String: CachedHTMLResponse]
+
+    // MARK: Initializers
+
+    /// Renders the document once per supported language.
+    /// - Parameters:
+    ///   - status: the status applied to every response. Defaults to `.ok`.
+    ///   - document: builds the document to render for a given locale.
+    init<Document: HTMLDocument>(
+        status: HTTPResponse.Status = .ok,
+        document: (Locale) -> Document
+    ) {
+        self.list = .init(bundle: .module)
+        self.responses = list.all
+            .reduce(into: [:]) { responses, language in
+                responses[language] = CachedHTMLResponse(
+                    status: status,
+                    additionalHeaders: [
+                        .contentLanguage: language,
+                        .vary: "Accept-Language",
+                    ],
+                    document: document(.init(identifier: language))
+                )
+            }
+    }
+
+    // MARK: Methods
+
+    /// Builds the response for the given language, falling back to the default language.
+    /// - Parameter language: the negotiated language identifier.
+    /// - Returns: the cached response for the language, the default language's response when the
+    ///   language is unavailable, or a `500 Internal Server Error` if neither is cached.
+    func response(
+        for language: String
+    ) -> Response {
+        guard
+            let response = responses[language] ?? responses[list.default]
+        else {
+            return .init(status: .internalServerError)
+        }
+
+        return response.response()
+    }
+
+}