The landing, privacy, terms, posts and profiles exist once per language at one path each: the default language at the clean URL, the others under their slug (/privacy, /de/privacy, /fr/privacy). Each is its own indexable page with a self canonical, hreflang for every language plus x-default, og:locale, <html lang> and a share image in its language; share links carry their language without a query. The route files are not duplicated: they moved under routes/{-$locale}/, TanStack Router's optional segment (SEO and discovery, commit).
The languages, their URL slugs and optional translated segments are one leaf, packages/intl/src/locale.config.ts; a project maps pt-BR to /br there, and the pure path functions in locale-paths.ts (split, localize, canonical form, region-tag matching for ?lang= and Accept-Language) read it. getSSRLanguage takes the language from the path on public pages and from the cookie everywhere else (Internationalization, commit).
The old URLs keep working: ?lang=de, a region tag (?lang=de-DE, ?lang=de_AT), /en/privacy and /DE/privacy answer a 301 to the path form with the rest of the query kept; an unknown language goes to the default page. A first human visit to / follows the locale cookie, then Accept-Language, with a 302 and Vary: Accept-Language, Cookie; crawlers, requests without Accept-Language, prefixed URLs and POST are never redirected (rootLocaleRedirect in the config). The language switcher on public pages is a list of links to the current page in each language and remembers the choice in the cookie; the header, footer, legal links and consent banner link within the page's language, and links to sign-in and the app stay unprefixed. The Markdown copies follow the same paths (/de/privacy.md; Accept: text/markdown on /de returns /de/index.md), the sitemap lists every language's URL with xhtml:link alternates and llms.txt names the per-language documents (Agent-readable pages).