Bundle and fallback english localization strings (#2430)

src/i18n.ts

@@ -2,6 +2,7 @@ import i18n from "i18next";
 import LanguageDetector from "i18next-browser-languagedetector";
 import resourcesToBackend from "i18next-resources-to-backend";
 import { initReactI18next } from "react-i18next";
+import enResources from "./Localization/en/Resources.json";
 
 i18n
   .use(LanguageDetector)
@@ -11,6 +12,19 @@ i18n
     fallbackLng: "en",
     defaultNS: "Resources",
     ns: ["Resources"],
+    // Statically bundle English resources so they are available synchronously on
+    // the very first render. Without this the async dynamic-import may not resolve
+    // in time (especially in incognito / cold-cache scenarios) and components would
+    // briefly show raw translation keys instead of translated strings.
+    resources: {
+      en: {
+        Resources: enResources,
+      },
+    },
+    // Allow the resources-to-backend plugin to still load other languages and
+    // namespaces dynamically; `partialBundledLanguages` tells i18next that only
+    // some languages are provided statically.
+    partialBundledLanguages: true,
     detection: { order: ["navigator", "cookie", "localStorage", "sessionStorage", "querystring", "htmlTag"] },
     debug: process.env.NODE_ENV === "development",
     keySeparator: ".",