persist locale on language switch when functional consent given

src/modules/i18n/localeMiddleware.ts

@@ -21,7 +21,7 @@ type MiddlewareRequest = {
  */
 export type LocaleMiddlewareResult =
   | { cookie?: { name: string; value: string }; location: string; type: 'redirect' }
-  | { type: 'next' }
+  | { cookie?: { name: string; value: string }; type: 'next' }
 
 type CreateLocaleMiddlewareArgs = {
   config: I18nConfig
@@ -66,10 +66,10 @@ function firstSegment(pathname: string): string {
  * import { localeMiddleware } from './ipal.middleware' // created from this factory
  * export function proxy(req) {
  *   const r = localeMiddleware(req)
- *   if (r.type === 'next') return NextResponse.next()
- *   const res = NextResponse.redirect(r.location)
- *   // cookie is optional: only present when the visitor consented to the
- *   // gating category (functional by default). Guard before setting.
+ *   // Both results may carry an optional cookie — 'next' when the visitor
+ *   // switched language (URL locale differs from the stored one) and consented,
+ *   // 'redirect' on the initial locale negotiation. Set it whenever present.
+ *   const res = r.type === 'next' ? NextResponse.next() : NextResponse.redirect(r.location)
  *   if (r.cookie) res.cookies.set(r.cookie.name, r.cookie.value)
  *   return res
  * }
@@ -80,11 +80,29 @@ export function createLocaleMiddleware({
   consentCookieName = CONSENT_COOKIE,
   cookieName = LOCALE_COOKIE_NAME,
 }: CreateLocaleMiddlewareArgs) {
+  // Whether the locale cookie may be written: 'necessary' is always granted;
+  // 'functional' (default) requires the visitor to have consented.
+  function mayPersistLocale(request: MiddlewareRequest): boolean {
+    if (consentCategory === 'necessary') {return true}
+    const consent = parseConsent(request.cookies.get(consentCookieName)?.value)
+    return consent?.[consentCategory] === true
+  }
+
   return function localeMiddleware(request: MiddlewareRequest): LocaleMiddlewareResult {
     const { pathname } = request.nextUrl
 
-    // Already locale-prefixed → nothing to do
-    if (isValidLocale(firstSegment(pathname), config)) {
+    // Already locale-prefixed (e.g. the visitor switched language by
+    // navigating to /en). Routing is fine — but if the URL's locale differs
+    // from the stored cookie, the visitor is *choosing* a language, and we
+    // should remember it — provided they consented to the gating category.
+    // Without consent we leave the cookie untouched: the switch works for this
+    // visit but isn't persisted, which is exactly the functional-cookie rule.
+    const urlLocale = firstSegment(pathname)
+    if (isValidLocale(urlLocale, config)) {
+      const currentCookie = request.cookies.get(cookieName)?.value ?? null
+      if (currentCookie !== urlLocale && mayPersistLocale(request)) {
+        return { type: 'next', cookie: { name: cookieName, value: urlLocale } }
+      }
       return { type: 'next' }
     }
 
@@ -106,16 +124,10 @@ export function createLocaleMiddleware({
     // Without consent the locale is still detected each request (routing works),
     // it just isn't remembered across visits — which is the whole point of
     // gating a functional cookie behind consent.
-    let mayPersist = consentCategory === 'necessary'
-    if (!mayPersist) {
-      const consent = parseConsent(request.cookies.get(consentCookieName)?.value)
-      mayPersist = consent?.[consentCategory] === true
-    }
-
     return {
       type: 'redirect',
       location: url.toString(),
-      ...(mayPersist ? { cookie: { name: cookieName, value: locale } } : {}),
+      ...(mayPersistLocale(request) ? { cookie: { name: cookieName, value: locale } } : {}),
     }
   }
 }