nginx: berlin: Normalize Accept-Language language code zh to zh-CN.

Now web browsers requesting any kind of Chinese get the website in
mainland Chinese.

zh, zh-Hans, zh-Hans-CN all are synonymous with zh-CN now.

Fixes <https://bugs.gnu.org/46807>.

* hydra/nginx/berlin.scm (accept-languages): New procedure.
(%extra-content): Normalize $lang variable with it.

hydra/nginx/berlin.scm

@@ -995,12 +995,37 @@ PUBLISH-URL."
        (uri "~ /(.*)")
        (body (list "return 301 $scheme://guixwl.org/$1;"))))))))
 
+(define (accept-languages language-lists)
+  "Returns nginx configuration code to set up the $lang variable
+according to the Accept-Language header in the HTTP request.  The
+requesting user agent will be served the files at /$lang/some/url.
+Each list in LANGUAGE-LISTS starts with the $lang and is followed by
+synonymous IETF language tags that should be mapped to the same $lang."
+  (define (language-mappings language-list)
+    (define (language-mapping language)
+      (string-join (list "    "  language (car language-list) ";")))
+    (string-join (map language-mapping language-list) "\n"))
+
+  (let ((directives
+         `(,(string-join
+             `("set_from_accept_language $lang_unmapped"
+               ,@(map string-join language-lists)
+               ";"))
+           "map $lang_unmapped $lang {"
+           ,@(map language-mappings language-lists)
+           "}")))
+    (string-join directives "\n")))
+
 (define %extra-content
   (list
    "default_type  application/octet-stream;"
    "sendfile        on;"
 
-   "set_from_accept_language $lang en de es fr zh-CN;"
+   (accept-languages '(("en")
+                       ("de")
+                       ("es")
+                       ("fr")
+                       ("zh-CN" "zh" "zh-Hans" "zh-Hans-CN")))
 
    ;; Maximum chunk size to send.  Partly this is a workaround for
    ;; <http://bugs.gnu.org/19939>, but also the nginx docs mention that