Load the Intl.Segmenter and Intl.DurationFormat polyfills only if needed (#2778)

* Load the Intl.Segmenter polyfill only if needed

* Also polyfill Intl.DurationFormat only if needed

* Polyfill Intl.* in tests

* Load the default translations in tests

* Instanciate the Intl.DurationFormat in the component

src/vitest.setup.ts

@@ -6,6 +6,8 @@ Please see LICENSE in the repository root for full details.
 */
 
 import "global-jsdom/register";
+import "@formatjs/intl-durationformat/polyfill";
+import "@formatjs/intl-segmenter/polyfill";
 import i18n from "i18next";
 import posthog from "posthog-js";
 import { initReactI18next } from "react-i18next";
@@ -14,6 +16,7 @@ import { cleanup } from "@testing-library/react";
 import "vitest-axe/extend-expect";
 import { logger } from "matrix-js-sdk/src/logger";
 
+import EN_GB from "../public/locales/en-GB/app.json";
 import { Config } from "./config/Config";
 
 // Bare-minimum i18n config
@@ -22,6 +25,13 @@ i18n
   .init({
     lng: "en-GB",
     fallbackLng: "en-GB",
+    supportedLngs: ["en-GB"],
+    // We embed the translations, so that it never needs to fetch
+    resources: {
+      "en-GB": {
+        app: EN_GB,
+      },
+    },
     interpolation: {
       escapeValue: false, // React has built-in XSS protections
     },