Enable overridding Content-Language per-response

Author: Sam Gammon

java/gust/backend/AppController.java

@@ -113,7 +113,11 @@ protected AppController(@Nonnull PageContextManager context) {
   protected @Nonnull <T> MutableHttpResponse<T> affixHeaders(@Nonnull MutableHttpResponse<T> response,
                                                              @Nonnull DynamicServingConfiguration config) {
     // first up: content language
-    if (config.language().isPresent()) {
+    if (this.context.language().isPresent()) {
+      var lang = this.context.language().get();
+      logging.debug(format("Affixing `Content-Language` header from context: '%s'.", lang));
+      response.setAttribute("language", lang);
+    } else if (config.language().isPresent()) {
       logging.debug(format("Affixing `Content-Language` header from config: '%s'.", config.language().get()));
       response.setAttribute("language", config.language().get());
       this.context.language(config.language());

java/gust/backend/PageContextManager.java

@@ -1494,6 +1494,19 @@ public boolean translate() {
     return this;
   }
 
+  /**
+   * Return the language value set for the current render routine - i.e. bound to the current request cycle. This is
+   * often driven by the user's browser settings.
+   *
+   * @return Current language for this request cycle.
+   */
+  public @Nonnull Optional<String> language() {
+    if (this.context.getLanguage().length() > 0) {
+      return Optional.of(this.context.getLanguage());
+    }
+    return Optional.empty();
+  }
+
   /**
    * Append an HTTP request header considered as part of the {@code Vary} header in the response. These values are de-
    * duplicated before joining and affixing.