add tests for i18n

Covers setLang/t language switching and fallback behavior, and
detectLang's navigator.language parsing — previously untested.

src/i18n.test.ts

@@ -0,0 +1,61 @@
+import { describe, it, expect, beforeEach, vi, afterEach } from "vitest";
+import { setLang, t, detectLang } from "./i18n";
+
+describe("i18n", () => {
+
+  beforeEach(() => {
+
+    const mockElements = [
+      { dataset: { i18n: "settings" }, textContent: "" },
+      { dataset: { i18n: "theme" }, textContent: "" }
+    ];
+
+    vi.stubGlobal("document", {
+      documentElement: { lang: "" },
+      querySelectorAll: vi.fn().mockReturnValue(mockElements)
+    });
+    vi.stubGlobal("navigator", { language: "en-US" });
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("sets language and updates document lang", () => {
+    setLang("ru");
+    expect(t("settings")).toBe("Настройки");
+    expect(document.documentElement.lang).toBe("ru");
+
+    // Check if querySelectorAll was called and elements were updated
+    const elements = document.querySelectorAll("[data-i18n]");
+    expect(elements[0].textContent).toBe("Настройки");
+    expect(elements[1].textContent).toBe("Тема");
+
+    setLang("en");
+    expect(t("settings")).toBe("Settings");
+    expect(document.documentElement.lang).toBe("en");
+  });
+
+  it("falls back to en for unknown languages", () => {
+    setLang("fr");
+    expect(t("settings")).toBe("Settings");
+    expect(document.documentElement.lang).toBe("en");
+  });
+
+  it("falls back to en dictionary for missing keys in ru", () => {
+    // If a key exists in en but not ru, it should fallback to en
+    // we don't have one right now, but we can test missing keys in both
+    expect(t("unknown_key")).toBe("unknown_key");
+  });
+
+  it("detects language from navigator", () => {
+    vi.stubGlobal("navigator", { language: "ru-RU" });
+    expect(detectLang()).toBe("ru");
+
+    vi.stubGlobal("navigator", { language: "en-US" });
+    expect(detectLang()).toBe("en");
+
+    vi.stubGlobal("navigator", { language: undefined });
+    expect(detectLang()).toBe("en");
+  });
+});