Add support for overriding strings in the app (#7886)

* Add support for overriding strings in the app

This is to support a case where certain details of the app need to be slightly different and don't necessarily warrant a complete fork. 

Intended for language-controlled deployments, operators can specify a JSON file with their custom translations that override the in-app/community-supplied ones.

* Fix import grouping

* Add a language handler test

* Appease the linter

* Add comment for why a weird class exists

test/languageHandler-test.ts

@@ -0,0 +1,70 @@
+/*
+Copyright 2022 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 SdkConfig from "../src/SdkConfig";
+import {
+    _t,
+    CustomTranslationOptions,
+    ICustomTranslations,
+    registerCustomTranslations,
+    setLanguage,
+} from "../src/languageHandler";
+
+describe('languageHandler', () => {
+    afterEach(() => {
+        SdkConfig.unset();
+        CustomTranslationOptions.lookupFn = undefined;
+    });
+
+    it('should support overriding translations', async () => {
+        const str = "This is a test string that does not exist in the app.";
+        const enOverride = "This is the English version of a custom string.";
+        const deOverride = "This is the German version of a custom string.";
+        const overrides: ICustomTranslations = {
+            [str]: {
+                "en": enOverride,
+                "de": deOverride,
+            },
+        };
+
+        const lookupUrl = "/translations.json";
+        const fn = (url: string): ICustomTranslations => {
+            expect(url).toEqual(lookupUrl);
+            return overrides;
+        };
+
+        // First test that overrides aren't being used
+
+        await setLanguage("en");
+        expect(_t(str)).toEqual(str);
+
+        await setLanguage("de");
+        expect(_t(str)).toEqual(str);
+
+        // Now test that they *are* being used
+        SdkConfig.add({
+            custom_translations_url: lookupUrl,
+        });
+        CustomTranslationOptions.lookupFn = fn;
+        await registerCustomTranslations();
+
+        await setLanguage("en");
+        expect(_t(str)).toEqual(enOverride);
+
+        await setLanguage("de");
+        expect(_t(str)).toEqual(deOverride);
+    });
+});