fix: Make locale cookie a session cookie (#1634)

Initially, I planned to add an expiration of 5 hours to the locale
cookie for `next-intl@4` to comply with GDPR regulations. However, this
has the implication that if the browser remains open for longer than 5
hours, the cookie can be reset in the middle of a session.

Due to this, it seems more reasonable to not set an expiration at all,
turning the cookie into a session cookie. Session cookies expiry only
when a browser is closed. On mobile, this can be even more beneficial,
as browsers are rarely closed (the browser can clear cookies though if
memory is constrained).

Author: amannn

docs/src/pages/docs/routing.mdx

@@ -473,11 +473,10 @@ In this case, only the locale prefix and a potentially [matching domain](#domain
 
 ### Locale cookie [#locale-cookie]
 
-If a user changes the locale to a value that doesn't match the `accept-language` header, `next-intl` will set a cookie called `NEXT_LOCALE` that contains the most recently detected locale. This is used to [remember the user's locale](/docs/routing/middleware#locale-detection) preference for future requests.
+If a user changes the locale to a value that doesn't match the `accept-language` header, `next-intl` will set a session cookie called `NEXT_LOCALE` that contains the most recently detected locale. This is used to [remember the user's locale](/docs/routing/middleware#locale-detection) preference for subsequent requests.
 
 By default, the cookie will be configured with the following attributes:
 
-1. [**`maxAge`**](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#max-agenumber): This value is set to 5 hours in order to be [GDPR-compliant](#locale-cookie-gdpr) out of the box.
 2. [**`sameSite`**](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#samesitesamesite-value): This value is set to `lax` so that the cookie can be set when coming from an external site.
 3. [**`path`**](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#pathpath-value): This value is not set by default, but will use the value of your [`basePath`](#base-path) if configured.
 
@@ -514,9 +513,9 @@ export const routing = defineRouting({
 <Details id="locale-cookie-gdpr">
 <summary>Which `maxAge` value should I consider for GDPR compliance?</summary>
 
-The [Article 29 Working Party opinion 04/2012](https://ec.europa.eu/justice/article-29/documentation/opinion-recommendation/files/2012/wp194_en.pdf) provides a guideline for the expiration of cookies that are used to remember the user's language in section 3.6 "UI customization cookies".
+The [Article 29 Working Party opinion 04/2012](https://ec.europa.eu/justice/article-29/documentation/opinion-recommendation/files/2012/wp194_en.pdf) provides a guideline for the expiration of cookies that are used to remember the user's language in section 3.6 "UI customization cookies". In this policy, a language preference cookie set as a result of an explicit user action, such as using a language switcher, is allowed to remain active for "a few additional hours" after a browser session has ended.
 
-In this policy, a language preference cookie set as a result of an explicit user action, such as using a language switcher, is allowed to remain active for "a few additional hours" after a browser session has ended. To be compliant out of the box, `next-intl` sets the `maxAge` value of the cookie to 5 hours.
+To be compliant out of the box, `next-intl` does not set the `max-age` value of the cookie, making it a session cookie that expires when a browser is closed.
 
 However, the Working Party also states that if additional information about the use of cookies is provided in a prominent location (e.g. a "uses cookies" notice next to the language switcher), the cookie can be configured to remember the user's preference for "a longer duration". If you're providing such a notice, you can consider increasing `maxAge` accordingly.
 

examples/example-app-router/tests/main.spec.ts

@@ -95,8 +95,6 @@ it("sets a cookie when requesting a locale that doesn't match the `accept-langua
   expect(value).toContain('NEXT_LOCALE=de;');
   expect(value).toContain('Path=/;');
   expect(value).toContain('SameSite=lax');
-  expect(value).toContain('Max-Age=18000;');
-  expect(value).toContain('Expires=');
 });
 
 it('serves a robots.txt', async ({page}) => {

packages/next-intl/src/navigation/react-client/createNavigation.test.tsx

@@ -273,8 +273,8 @@ describe("localePrefix: 'always', with `localeCookie`", () => {
       expect(cookieSpy).toHaveBeenCalledWith(
         [
           'NEXT_LOCALE=de',
-          'max-age=60',
           'sameSite=strict',
+          'max-age=60',
           'domain=example.com',
           'partitioned',
           'path=/nested',
@@ -297,8 +297,8 @@ describe("localePrefix: 'always', with `localeCookie`", () => {
       expect(cookieSpy).toHaveBeenCalledWith(
         [
           'NEXT_LOCALE=de',
-          'max-age=60',
           'sameSite=strict',
+          'max-age=60',
           'domain=example.com',
           'partitioned',
           'path=/nested',
@@ -345,12 +345,7 @@ describe("localePrefix: 'always', with `basePath`", () => {
       invokeRouter((router) => router.push('/about', {locale: 'de'}));
 
       expect(cookieSpy).toHaveBeenCalledWith(
-        [
-          'NEXT_LOCALE=de',
-          'max-age=18000',
-          'sameSite=lax',
-          'path=/base/path'
-        ].join(';') + ';'
+        ['NEXT_LOCALE=de', 'sameSite=lax', 'path=/base/path'].join(';') + ';'
       );
       cookieSpy.mockRestore();
     });

packages/next-intl/src/routing/config.tsx

@@ -157,7 +157,6 @@ function receiveLocaleCookie(
   return (localeCookie ?? true)
     ? {
         name: 'NEXT_LOCALE',
-        maxAge: 5 * 60 * 60, // 5 hours
         sameSite: 'lax',
         ...(typeof localeCookie === 'object' && localeCookie)
 
@@ -173,7 +172,7 @@ export type LocaleCookieConfig = Omit<
   CookieAttributes,
   'name' | 'maxAge' | 'sameSite'
 > &
-  Required<Pick<CookieAttributes, 'name' | 'maxAge' | 'sameSite'>>;
+  Required<Pick<CookieAttributes, 'name' | 'sameSite'>>;
 
 function receiveLocalePrefixConfig<
   AppLocales extends Locales,