feat: unify language handling across the application

- Updated the language configuration to use a single source of truth from `DEFAULT_LANGUAGE` for the bot, API, and Mini App, eliminating auto-detection from user settings.
- Refactored the `get_lang` function to always return `DEFAULT_LANGUAGE`, ensuring consistent language usage throughout the application.
- Modified the handling of language in various components, including API responses and UI elements, to reflect the new language management approach.
- Enhanced documentation and comments to clarify the changes in language handling.
- Added unit tests to verify the new language handling behavior and ensure coverage for the updated functionality.

webapp/js/i18n.test.js

@@ -1,50 +1,76 @@
 /**
- * Unit tests for i18n: getLang, t (fallback, params), monthName.
+ * Unit tests for i18n: getLang (window.__DT_LANG), normalizeLang, t (fallback, params), monthName.
  */
 
-import { describe, it, expect, vi, beforeEach } from "vitest";
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
 
-const mockGetInitData = vi.fn();
-vi.mock("./auth.js", () => ({ getInitData: () => mockGetInitData() }));
-
-import { getLang, t, monthName, MESSAGES } from "./i18n.js";
+import { getLang, normalizeLang, t, monthName, MESSAGES } from "./i18n.js";
 
 describe("getLang", () => {
-  const origNavigator = globalThis.navigator;
+  const orig__DT_LANG = globalThis.window?.__DT_LANG;
 
-  beforeEach(() => {
-    mockGetInitData.mockReset();
+  afterEach(() => {
+    if (typeof globalThis.window !== "undefined") {
+      if (orig__DT_LANG !== undefined) {
+        globalThis.window.__DT_LANG = orig__DT_LANG;
+      } else {
+        delete globalThis.window.__DT_LANG;
+      }
+    }
   });
 
-  it("returns lang from initData user when present", () => {
-    mockGetInitData.mockReturnValue(
-      "user=" + encodeURIComponent(JSON.stringify({ language_code: "en" }))
-    );
-    expect(getLang()).toBe("en");
-  });
-
-  it("normalizes ru from initData", () => {
-    mockGetInitData.mockReturnValue(
-      "user=" + encodeURIComponent(JSON.stringify({ language_code: "ru" }))
-    );
+  it("returns ru when window.__DT_LANG is ru", () => {
+    globalThis.window.__DT_LANG = "ru";
     expect(getLang()).toBe("ru");
   });
 
-  it("falls back to navigator.language when initData empty", () => {
-    mockGetInitData.mockReturnValue("");
-    Object.defineProperty(globalThis, "navigator", {
-      value: { ...origNavigator, language: "en-US", languages: ["en-US", "en"] },
-      configurable: true,
-    });
+  it("returns en when window.__DT_LANG is en", () => {
+    globalThis.window.__DT_LANG = "en";
     expect(getLang()).toBe("en");
   });
 
-  it("normalizes to en for unknown language code", () => {
-    mockGetInitData.mockReturnValue(
-      "user=" + encodeURIComponent(JSON.stringify({ language_code: "uk" }))
-    );
+  it("returns en when window.__DT_LANG is missing", () => {
+    delete globalThis.window.__DT_LANG;
     expect(getLang()).toBe("en");
   });
+
+  it("returns en when window.__DT_LANG is invalid (unknown code)", () => {
+    globalThis.window.__DT_LANG = "uk";
+    expect(getLang()).toBe("en");
+  });
+
+  it("returns ru when window.__DT_LANG is ru-RU (normalized)", () => {
+    globalThis.window.__DT_LANG = "ru-RU";
+    expect(getLang()).toBe("ru");
+  });
+
+  it("returns en when window.__DT_LANG is empty string", () => {
+    globalThis.window.__DT_LANG = "";
+    expect(getLang()).toBe("en");
+  });
+
+  it("returns en when window.__DT_LANG is null", () => {
+    globalThis.window.__DT_LANG = null;
+    expect(getLang()).toBe("en");
+  });
+});
+
+describe("normalizeLang", () => {
+  it("returns ru for ru-like codes", () => {
+    expect(normalizeLang("ru")).toBe("ru");
+    expect(normalizeLang("ru-RU")).toBe("ru");
+  });
+
+  it("returns en for en and others", () => {
+    expect(normalizeLang("en")).toBe("en");
+    expect(normalizeLang("en-US")).toBe("en");
+    expect(normalizeLang("uk")).toBe("en");
+  });
+
+  it("returns en for empty or invalid", () => {
+    expect(normalizeLang("")).toBe("en");
+    expect(normalizeLang(null)).toBe("en");
+  });
 });
 
 describe("t", () => {