feat(i18n): an application uses Core's translator and words Core's messages its own way

The ./i18n entry exports t, tn, msg, translateKnown and the number and date
formatters. setMessageOverrides({ en: {...}, de: {...} }) shows the
application's text for any catalog key: in the active locale, else its
English, else Core's translation.

docs/I18N.md

@@ -83,6 +83,21 @@ menu.onChoose = (code) => switchLocale(code); // switched in place and stored (i
 
 `switchLocale` does what a choice in the globe picker does, and returns `'unchanged'` for the active language. `onLocaleChange` tells the application's own menu to mark the new language.
 
+The same entry exports the translator Core's text goes through (`t`, `tn`, `msg`, `translateKnown`, `formatNumber`, `formatDate`, `formatRelativeTime`), so the application's own text follows the same language. To show its own wording for one of Core's messages, it overrides it by the catalog key, in English and in any locale it words:
+
+```js
+import { setMessageOverrides } from '<core package>/i18n';
+
+setMessageOverrides({
+  en: { 'Download scene JSON': 'Download scene file' },
+  de: { 'Download scene JSON': 'Szenendatei herunterladen' },
+});
+```
+
+Keys are the catalogs' (`context|text` for a context, the English plural for `tn`). A `t` entry is text; a `tn` entry is `{one, other, …}` forms, chosen by the plural rules of the locale they are worded in; an entry of the other kind is ignored. An overridden message shows the application's text in the active locale, else its English: an English-only override replaces Core's translations of that message too, so word every locale that should keep its own. Each call replaces the earlier overrides and `{}` removes them.
+
+Call it before views render. `onLocaleChange` listeners word their text again at once, but a view already drawn keeps its text until it next renders. Import it from the `./i18n` entry only: a second copy of the module (a path alias into Core's sources) keeps its own overrides and language.
+
 ## Rules
 
 - **Pass literals.** `t('Close')`, not `t(label)` (unless `label` holds a `msg()` value) and never a template literal with `${…}`.