Use Intl for names of languages (#11427)

* Use Intl for names of languages

* Tweak Intl language style from "American English" -> "US English"

* Update tests

* Fix tests

* Consolidate languageHandler-test files

* Improve coverage

* Consistent casing for languages in dropdown

* Update LanguageDropdown.tsx

* Delint & update snapshot

* Fix tests

* Improve coverage

`of` will fallback to the given code with fallback=code (default)

test/components/views/elements/SpellCheckLanguagesDropdown-test.tsx

@@ -0,0 +1,38 @@
+/*
+Copyright 2023 The Matrix.org Foundation C.I.C.
+
+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 React from "react";
+import { render, screen, waitForElementToBeRemoved } from "@testing-library/react";
+
+import SpellCheckLanguagesDropdown from "../../../../src/components/views/elements/SpellCheckLanguagesDropdown";
+import PlatformPeg from "../../../../src/PlatformPeg";
+
+describe("<SpellCheckLanguagesDropdown />", () => {
+    it("renders as expected", async () => {
+        const platform: any = { getAvailableSpellCheckLanguages: jest.fn().mockResolvedValue(["en", "de", "qq"]) };
+        PlatformPeg.set(platform);
+
+        const { asFragment } = render(
+            <SpellCheckLanguagesDropdown
+                className="mx_GeneralUserSettingsTab_spellCheckLanguageInput"
+                value="en"
+                onOptionChange={jest.fn()}
+            />,
+        );
+        await waitForElementToBeRemoved(() => screen.queryAllByLabelText("Loading…"));
+        expect(asFragment()).toMatchSnapshot();
+    });
+});