The UI language follows the device (the local choice / system guess) and is
deliberately not overridden from the account, but the advertising banner and
out-of-app push routing are resolved server-side from preferred_language. A
saved device choice the account had not recorded — picked while a guest, or
differing from the Telegram system-language seed — left the banner (and pushes)
in the wrong language until a Settings change rewrote preferred_language.
On profile load (adoptSession and the in-place link path) push the saved local
choice to the account when it differs (new pure helper languageNeedsServerSync;
no-op for guests and when already equal), so the banner and pushes match the
visible UI from the first open. Unit-tested.
Ilia Denisov