Add test for the language switcher

frontend/src/components/LanguageSwitcher/LanguageSwitcher.test.tsx

@@ -0,0 +1,37 @@
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { Language } from "../../lib/types";
+import LanguageSwitcher from "./LanguageSwitcher";
+
+const renderSwitcher = (onChange = () => {}, defaultLang = Language.ENG) => {
+  render(<LanguageSwitcher defaultLang={defaultLang} onChange={onChange} />);
+};
+
+test("renders LanguageSwitcher component", async () => {
+  renderSwitcher();
+  const switcher = screen.getByTestId("language-switcher") as HTMLSelectElement;
+
+  expect(switcher.value).toBe(Language.ENG);
+});
+
+test("calls onChange handler when a new language is selected", async () => {
+  const mockOnChange = jest.fn();
+  renderSwitcher(mockOnChange);
+
+  const switcher = screen.getByTestId("language-switcher") as HTMLSelectElement;
+
+  await userEvent.selectOptions(switcher, Language.ESP);
+  expect(mockOnChange).toHaveBeenCalledWith(Language.ESP);
+});
+
+test("renders all language options correctly", () => {
+  renderSwitcher();
+  const options = screen.getAllByRole("option");
+  const languageValues = Object.values(Language);
+
+  expect(options).toHaveLength(languageValues.length);
+  languageValues.forEach((lang, index) => {
+    expect(options[index]).toHaveValue(lang);
+    expect(options[index]).toHaveTextContent(lang.toUpperCase());
+  });
+});

frontend/src/components/LanguageSwitcher/LanguageSwitcher.tsx

@@ -10,6 +10,7 @@ function LanguageSwitcher({
 }) {
   return (
     <select
+      data-testid="language-switcher"
       defaultValue={defaultLang}
       onChange={(ev) => onChange(ev.currentTarget.value as Language)}
       className={styles.languageSwitcher}