diff --git a/README.md b/README.md
index 095f26f..6fe1865 100644
--- a/README.md
+++ b/README.md
@@ -83,7 +83,8 @@ To Be Played is split into three repositories:
 - **Appearance**: accent color, corner style, wide layout, page background and banner from your own game art.
   Saved in Auth, so it follows you to every device.
 - **Language**: kept in the browser and saved in Auth; after the login Auth's language wins when one is saved. Dark
-  mode stays per device.
+  mode stays per device. The registration sends the browser language (`de` for a German browser, else `en`): Auth
+  writes its mails in it and saves it as the user's language.
 - **Broken pictures**: a Steam cover or achievement icon that does not load shows a placeholder and is reported
   to Core once per page load (`POST /api/library/broken-images`, only with a session). Core repairs it at night;
   a repaired cover comes back as `imageUrl`.
diff --git a/src/pages/RegistrationPage.jsx b/src/pages/RegistrationPage.jsx
index 9204c4d..8340be1 100644
--- a/src/pages/RegistrationPage.jsx
+++ b/src/pages/RegistrationPage.jsx
@@ -54,6 +54,8 @@ export default function RegistrationPage() {
                     password: values.password,
                     inviteCode: values.inviteCode,
                     displayName: values.displayName,
+                    // Auth's mails use it: a German browser gets German, every other one English.
+                    language: navigator.language?.toLowerCase().startsWith('de') ? 'de' : 'en',
                 }),
             });
 
diff --git a/src/pages/RegistrationPage.test.jsx b/src/pages/RegistrationPage.test.jsx
index fd73e88..9cf21e2 100644
--- a/src/pages/RegistrationPage.test.jsx
+++ b/src/pages/RegistrationPage.test.jsx
@@ -30,6 +30,7 @@ function submitValidForm() {
 describe('RegistrationPage', () => {
     afterEach(() => {
         cleanup();
+        vi.restoreAllMocks();
     });
 
     it('shows the wait time when the IP is blocked', async () => {
@@ -50,6 +51,17 @@ describe('RegistrationPage', () => {
         expect(JSON.parse(fetch.mock.calls[0][1].body)).not.toHaveProperty('steamId');
     });
 
+    it.each([['de-DE', 'de'], ['de', 'de'], ['en-US', 'en'], ['fr-FR', 'en'], [undefined, 'en']])(
+        'sends the browser language %s as %s, for the mails', async (browser, sent) => {
+            vi.spyOn(navigator, 'language', 'get').mockReturnValue(browser);
+            answerRegistration(400, 'INVALID_INVITATION_CODE');
+
+            submitValidForm();
+
+            await screen.findByText('REGISTRATION.ERROR_INVALID_INVITE_CODE');
+            expect(JSON.parse(fetch.mock.calls[0][1].body).language).toBe(sent);
+        });
+
     it('maps a known backend error to its message', async () => {
         answerRegistration(400, 'INVALID_INVITATION_CODE');