Rename en-GB language to en (#2866)

Fixes https://github.com/element-hq/element-call/issues/2865

If we want to have GB and US specific terms later on we can then add then in as `en-GB` and `en-US`.

src/@types/i18next.d.ts

@@ -7,7 +7,7 @@ Please see LICENSE in the repository root for full details.
 
 import "i18next";
 // import all namespaces (for the default language, only)
-import app from "../../locales/en-GB/app.json";
+import app from "../../locales/en/app.json";
 
 declare module "i18next" {
   interface CustomTypeOptions {

src/TranslatedError.ts

@@ -22,7 +22,7 @@ export abstract class TranslatedError extends Error {
     messageKey: ParseKeys<DefaultNamespace, TOptions>,
     translationFn: TFunction<DefaultNamespace>,
   ) {
-    super(translationFn(messageKey, { lng: "en-GB" } as TOptions));
+    super(translationFn(messageKey, { lng: "en" } as TOptions));
     this.translatedMessage = translationFn(messageKey);
   }
 }

src/initializer.tsx

@@ -24,7 +24,7 @@ import { platform } from "./Platform";
 
 // This generates a map of locale names to their URL (based on import.meta.url), which looks like this:
 // {
-//   "../locales/en-GB/app.json": "/whatever/assets/root/locales/en-aabbcc.json",
+//   "../locales/en/app.json": "/whatever/assets/root/locales/en-aabbcc.json",
 //   ...
 // }
 const locales = import.meta.glob<string>("../locales/*/*.json", {
@@ -41,7 +41,7 @@ const getLocaleUrl = (
 const supportedLngs = [
   ...new Set(
     Object.keys(locales).map((url) => {
-      // The URLs are of the form ../locales/en-GB/app.json
+      // The URLs are of the form ../locales/en/app.json
       // This extracts the language code from the URL
       const lang = url.match(/\/([^/]+)\/[^/]+\.json$/)?.[1];
       if (!lang) {
@@ -133,7 +133,7 @@ export class Initializer {
       .use(languageDetector)
       .use(initReactI18next)
       .init({
-        fallbackLng: "en-GB",
+        fallbackLng: "en",
         defaultNS: "app",
         keySeparator: ".",
         nsSeparator: false,

src/vitest.setup.ts

@@ -17,20 +17,20 @@ import "vitest-axe/extend-expect";
 import { logger } from "matrix-js-sdk/src/logger";
 import "@testing-library/jest-dom/vitest";
 
-import EN_GB from "../locales/en-GB/app.json";
+import EN from "../locales/en/app.json";
 import { Config } from "./config/Config";
 
 // Bare-minimum i18n config
 i18n
   .use(initReactI18next)
   .init({
-    lng: "en-GB",
-    fallbackLng: "en-GB",
-    supportedLngs: ["en-GB"],
+    lng: "en",
+    fallbackLng: "en",
+    supportedLngs: ["en"],
     // We embed the translations, so that it never needs to fetch
     resources: {
-      "en-GB": {
-        app: EN_GB,
+      en: {
+        app: EN,
       },
     },
     interpolation: {