#179 : negotiate the SPA shell's lang attribute in nginx

`apps/web/index.html` carries a hard `lang="en"`. The app corrects it at runtime (#163), but nginx answers every SPA route with that same file, so a crawler or a no-JS visit — `/public/...` on prod is exactly that — saw `en` for German content, permanently. WCAG 3.1.1 is about the delivered document, not the one JavaScript later fixes. nginx-only, no backend involved: a `map` on `Accept-Language` and a `sub_filter` in the index.html path. Only the FIRST tag decides, which is what "the browser's preferred language" means and mirrors #163 — `de-CH` counts as German, `en-US,de` does not. `Vary: Accept-Language` is new. The response now genuinely depends on a request header, and without it a shared cache could hand one language's copy to the other. Everything else in the location is untouched: same CSP, same `Cache-Control: no-cache`, same `nosniff`. The known limit is documented in the config rather than worked around: nginx cannot read `instance.defaultLocale`, so an unlisted or absent Accept-Language yields `en` even on a German instance. For public content that is not the authoritative rendering anyway — the api's server shell (`/api/v1/public/...`) already renders those with the instance locale. Verified against a real nginx 1.27 (the image the stage runs) with the config mounted as-is: `de-DE,de;q=0.9,en;q=0.8` and `de` yield `lang="de"`; `en-US,en;q=0.9`, `en-US,en;q=0.9,de;q=0.8`, `fr-FR,fr` and a request with no header yield `lang="en"`; an SPA route (`/public/teich/seite`) negotiates the same way; the gzipped response is rewritten too, and a JavaScript asset comes through byte-identical.
2026-08-01 19:18:24 +02:00
1 changed files with 31 additions and 0 deletions
--- a/apps/web/nginx.conf
+++ b/apps/web/nginx.conf
@ -1,3 +1,23 @@
+# The SPA shell's `lang` attribute, negotiated from the request (issue #179,
+# WCAG 3.1.1). `apps/web/index.html` is a static file with a hard `lang="en"`;
+# the app corrects it at runtime (#163), but a crawler or a no-JS visit of an
+# SPA route — which nginx answers with index.html — would see `en` forever,
+# even for German content.
+#
+# Only the FIRST tag of Accept-Language decides, which is what "the browser's
+# preferred language" means and mirrors #163's semantics. `de-CH` counts as
+# German; `en-US,de` does not, because that visitor asked for English first.
+#
+# Known limit, documented rather than worked around: nginx does not know
+# `instance.defaultLocale` from the database, so a visitor with no (or an
+# unlisted) Accept-Language gets `en` even on a German instance. For PUBLIC
+# content that is not the authoritative rendering anyway — the api's server
+# shell (`/api/v1/public/...`) renders those with the instance locale.
+map $http_accept_language $spa_lang {
+    default en;
+    ~*^de    de;
+}
+
 # SPA serving: static assets with long-lived caching, everything else
 # falls back to index.html (client-side routing).
 server {
@ -34,6 +54,17 @@ server {
        # zero-third-party-request guarantee (security.md) is unaffected.
        add_header Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; font-src 'self' data:; img-src 'self' data: blob:; connect-src 'self'; worker-src 'self'; manifest-src 'self'; object-src 'none'; base-uri 'self'; frame-ancestors 'self'" always;
        add_header X-Content-Type-Options "nosniff" always;
+        # The shell's language (issue #179). Only the html document is
+        # rewritten, and only its first match — `<html lang="en">` is the
+        # first and only occurrence in index.html. Everything else this
+        # location serves passes through untouched.
+        sub_filter_types text/html;
+        sub_filter_once on;
+        sub_filter 'lang="en"' 'lang="$spa_lang"';
+        # The response now depends on a request header, so shared caches must
+        # not serve one language's copy to the other. This location is
+        # `no-cache` anyway; the header states the dependency correctly.
+        add_header Vary "Accept-Language" always;
        try_files $uri /index.html;
    }
 }