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.

src/app/controls.js

@@ -9,7 +9,8 @@ import { t } from '../i18n/index.js';
  * Construct the existing controls and camera presentation. `initialShareCode`
  * opens a share code (the `<code>` of `/share/<code>`) at start-up, for a host
  * whose address does not carry one. `localePicker: false` removes the language
- * picker, for a host that does not initialize Core's locale.
+ * picker, for a host that offers its own language choice (`switchLocale` from
+ * the `./i18n` entry).
  */
 export function createApplicationControls({
   scene: { viewer, mapStackController, operations },