Add Russian to the language site and the forum placeholderfeature/russian-locale

Russian (ru-RU) is the third locale of the language site: the shell, the
home, overview and getting-started pages, and the page metadata are fully
translated, and the language menu offers it. The documentation sidebar reads
its labels from the page messages instead of a two-way locale test, and the
stored preference is accepted only when it names a supported locale.

The forum placeholder gets the same three languages. Its language control
is now a menu instead of a two-state button, and an unknown stored or
selected value keeps the current language.

The site API lists the new locale. Verified locally: prettier, the frontend
tests, vue-tsc and the Vite build, the backend tests, and a browser run of
the home and overview pages in Russian.

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