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.

scripts/format-scope.json

@@ -1342,6 +1342,8 @@
   "src/data/layerState.js",
   "src/data/layerState.test.mjs",
   "src/data/spriteOrder.js",
+  "src/i18n/page.js",
+  "src/i18n/page.test.mjs",
   "src/i18n/locales/ar.json",
   "src/i18n/locales/de.json",
   "src/i18n/locales/es.json",