feat(i18n): an ./i18n entry for an application with its own language choice

startPageLocale, followPageLocale and switchLocale start, follow and switch
the page's language as the standalone application and the globe picker do,
for an application that offers the language elsewhere and passes
localePicker: false. The standalone application uses them.

docs/CURRENT-STATE.md

@@ -5080,6 +5080,10 @@ Shipped locales: **en** (source and fallback), **ar**, **de**, **es**, **fr**, *
   views re-render, code that published translated text words it again through
   `onLocaleChange`, and `<html lang>`/`dir` and the page's `data-i18n` markup
   follow. Nothing is lost; a toast already on screen keeps its words.
+  An application that offers the language elsewhere (a menu) uses the
+  `./i18n` entry (`src/i18n/page.js`): `startPageLocale` at boot,
+  `followPageLocale` for the markup and `switchLocale`, which does what a
+  picker choice does, with `localePicker: false`.
 - **Right to left.** Arabic (`ar`) and Hebrew (`he`) are in `RTL_LOCALES`, so
   `<html dir="rtl">`: the title, style indicator, both panel stacks and their
   icon rails swap sides through logical properties (`inset-inline-*`,