The language control no longer reloads the page. It calls a server action
that stores the preference cookie; Next.js then renders the current route
again and applies it in place, so scroll position, theme and client state
stay. The lang query parameter remains for links and for browsers without
script, and is the fallback when the action cannot reach the server. Both
paths write the cookie with the same attributes from one function.
The toggle button is now a language menu, a native select that lists each
language by its own name, and Russian (ru-RU) is the third locale with a
complete translation of every message.
Tests require every locale to have exactly the message shape of the default
locale with no empty text, and cover the cookie attributes and host
normalization. Verified locally: prettier, eslint, type check, tests, the
production build, and a browser run on the apex and ViGet hosts where the
language changed with no navigation and the scroll position kept.