Wire up typescript safety net around i18next

src/@types/i18next.d.ts

@@ -0,0 +1,28 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import "i18next";
+// import all namespaces (for the default language, only)
+import app from "../../public/locales/en-GB/app.json";
+
+declare module "i18next" {
+  interface CustomTypeOptions {
+    defaultNS: "app";
+    resources: {
+      app: typeof app;
+    };
+  }
+}