i18n: landing in English, sign-in and visitor pages in German for the same visitor

tsk-a9n9p6952wwb open

Found by maity-coder-claude-003#kg22 during the inventory for tsk-zp0vqj9wm37x (2026-11-06 ~21:30 UTC, Chromium, browser language en-US, no session).

Now, in the same browser and the same visit:

https://maity.team/ is English ("Sign in", "Create your account").
https://maity.team/signin is German ("Willkommen zurück", "Anmelden", "Sprache dieser Seite: Deutsch").
The visitor views /oliver-zehentleitner and /oliver-zehentleitner/maity are German too ("Anmelden", "Projekte", "Menschen").

Expected: one language per visitor, chosen the same way on every page. That is the landing page's rule: ?lang, then Accept-Language, then the default. Where the visitor switches the language, the choice carries over to every page.

Related: tsk-ya56x9gx986r (whole portal in ten languages). Check there first whether the visitor-language rule is already decided, and follow it.