Added language support to the Website service target.

Services/Website/Sources/Library/Internal/Extensions/Page+Defaults.swift

@@ -16,6 +16,14 @@ extension Page {
         []
     }
 
+    /// The territories paired with the site's languages, in Open Graph's `language_TERRITORY` form.
+    ///
+    /// English ships as `en_US`, Open Graph's conventional default. A site serving a language in a territory of its own repoints or extends the
+    /// map (`en_NL`, `nl_NL`, …); a language with no entry stays a bare code, which scrapers also accept.
+    static var ogLocales: [String: String] {
+        ["en": "en_US"]
+    }
+
     // MARK: Computed
 
     /// The document language, derived from the page's locale and falling back to the default language.
@@ -24,11 +32,65 @@ extension Page {
             ?? LanguageList().default
     }
 
+    /// The locale of the page's social card, in Open Graph's `language_TERRITORY` form where ``ogLocales`` pairs the page's language with a
+    /// territory; a language it does not name stays a bare code, which scrapers also accept.
+    ///
+    /// Keyed off ``lang``, so the card's locale and the document's `lang` attribute never describe the document differently. Nothing reads it
+    /// until a page supplies a `socialCard` — like ``preloadedFonts``, it is a hook a generated site fills in.
+    var ogLocale: String {
+        Self.ogLocales[lang] ?? lang
+    }
+
     /// The site-wide head metadata; a page that adds tags of its own composes ``siteMetadata`` rather than replacing it.
     var metadata: some HTML {
         siteMetadata
     }
 
+    /// The `hreflang` alternates tying the page's language editions together, or nothing without an origin — the annotations require absolute URLs.
+    ///
+    /// Nothing is emitted for a single-language site either: a set naming one edition tells a search engine nothing it cannot already see.
+    /// `x-default` points at the catalog's default language, whose bare URL negotiates the language and so is the right landing for everyone
+    /// unmatched — it stays the catalog's default even when `languages` names a subset.
+    /// - Parameters:
+    ///   - origin: the site's public origin, or `nil` to emit nothing.
+    ///   - path: the page's bare (default-language) path.
+    ///   - languages: the languages the page is published in; every catalog language by default. A page translated into only some of them
+    ///     narrows the set, so it never advertises an edition that does not exist.
+    /// - Returns: one `alternate` link per language, followed by the `x-default` link.
+    @HTMLBuilder
+    func languageAlternates(
+        origin: String?,
+        path: String,
+        languages: [Language] = Language.all
+    ) -> some HTML {
+        if let origin, languages.count > 1 {
+            ForEach(languages) { language in
+                link(
+                    .rel("alternate"),
+                    .custom(
+                        name: "hreflang",
+                        value: language.identifier
+                    ),
+                    .href(language.url(
+                        origin: origin,
+                        path: path
+                    ))
+                )
+            }
+            link(
+                .rel("alternate"),
+                .custom(
+                    name: "hreflang",
+                    value: "x-default"
+                ),
+                .href(Language.default.url(
+                    origin: origin,
+                    path: path
+                ))
+            )
+        }
+    }
+
     /// The ``preloadedFonts`` links, then the icon, manifest, and theme colour metadata shared by every page of the website.
     ///
     /// A preload URL must match the stylesheet's `@font-face` source exactly — unversioned, with `crossorigin` — or the browser fetches the