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/APPLICATION.md

@@ -224,7 +224,9 @@ leaving out `command-dock` the voice control and its pointer reticle;
 leaving out `context` the Bhote Koshi reconstruction panel; and leaving out
 `intel-hud` the Intel HUD overlay (`#intel-hud`), whose readouts the HUD
 implementation publishes to the `gev-hud` store. An unknown
-view name is an error. A host that does not initialize Core's locale passes
+view name is an error. An application that offers its own language choice
+starts and switches the language with the `./i18n` entry (`startPageLocale`,
+`followPageLocale`, `switchLocale`; see [I18N.md](./I18N.md)) and passes
 `localePicker: false` to `createApplicationControls` instead of editing the
 markup. Node tests that import application modules register the component
 loader with `node --import=<core package>/build/register-vue`.