Load the default translations in tests

src/vitest.setup.ts

@@ -16,6 +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
@@ -24,6 +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
     },