feat(registration): send the browser language

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`.