A user who never changed the language in Settings kept their account at the
creation-time preferred_language seed (e.g. en from the Telegram launch language_code)
even after switching the device to another language: the UI followed the device (ru) but
the ad banner and out-of-app push — both resolved server-side from preferred_language —
stayed en. The on-adopt reconcile was gated on an explicit local choice (localeLocked),
so a system-guess locale was never pushed through.
Reconcile preferred_language to the active interface locale (app.locale) on every session
adopt and link, regardless of how the locale was chosen; persistLanguageToServer already
self-gates (a no-op for guests and when already equal), so there is no steady-state write.
The banner and push are the only server-rendered language surfaces and both read
preferred_language, so this keeps the whole interface consistent — not just the banner.
Drop the now-dead localeLocked flag (the reconcile guards were its only readers; the saved
prefs.locale still restores the UI choice per device).
Trade-off: preferred_language now follows the most-recently-opened device, so an explicit
choice on one device can be overwritten by a system guess on another (the "explicit" mark
is local, per-device); making it globally sticky would need a DB flag.
Docs: ARCHITECTURE §4 + the profile field.
Ilia Denisov