feat(i18n): add language switcher

tests/e2e/language-switcher.spec.ts

@@ -0,0 +1,93 @@
+import { expect, type Page, test } from "@playwright/test"
+
+async function setLocaleCookie(
+  page: Page,
+  locale: "en" | "es",
+  baseURL?: string,
+) {
+  await page.context().addCookies([
+    {
+      name: "stock-manager-locale",
+      value: locale,
+      url: baseURL ?? "http://127.0.0.1:3100",
+    },
+  ])
+}
+
+async function expectLocaleCookie(page: Page, locale: "en" | "es") {
+  await expect
+    .poll(async () => {
+      const cookies = await page.context().cookies()
+      return cookies.find((cookie) => cookie.name === "stock-manager-locale")
+        ?.value
+    })
+    .toBe(locale)
+}
+
+async function signInAsAdmin(page: Page, baseURL?: string) {
+  await setLocaleCookie(page, "en", baseURL)
+  await page.goto("/login")
+  await page.getByLabel("Username").fill("admin")
+  await page.getByLabel("Password").fill("admin-password")
+  await page.getByRole("button", { name: "Sign In" }).click()
+  await expect(page).toHaveURL("/")
+}
+
+test.describe("language switcher", () => {
+  test("switches the login page language in place through the locale cookie", async ({
+    baseURL,
+    page,
+  }) => {
+    await setLocaleCookie(page, "en", baseURL)
+
+    await page.goto("/login")
+
+    await expect(page).toHaveURL(/\/login$/)
+    await expect(page.locator("html")).toHaveAttribute("lang", "en")
+    await expect(page.getByRole("heading", { name: "Sign In" })).toBeVisible()
+
+    await page.getByRole("button", { name: "Language: English" }).click()
+
+    const options = page.getByRole("menuitemradio")
+    await expect(options).toHaveCount(2)
+    await expect(
+      page.getByRole("menuitemradio", { name: "English" }),
+    ).toBeVisible()
+    await page.getByRole("menuitemradio", { name: "Spanish" }).click()
+
+    await expect(page).toHaveURL(/\/login$/)
+    await expect(page.locator("html")).toHaveAttribute("lang", "es")
+    await expect(
+      page.getByRole("heading", { name: "Iniciar sesión" }),
+    ).toBeVisible()
+    await expect(page.getByLabel("Usuario")).toBeVisible()
+    await expect(page.getByLabel("Contraseña")).toBeVisible()
+    await expect(
+      page.getByRole("button", { name: "Iniciar sesión" }),
+    ).toBeVisible()
+    await expectLocaleCookie(page, "es")
+  })
+
+  test("switches the authenticated dashboard language from the navbar", async ({
+    baseURL,
+    page,
+  }) => {
+    await signInAsAdmin(page, baseURL)
+
+    await expect(page.locator("html")).toHaveAttribute("lang", "en")
+    await expect(page.getByRole("heading", { name: "Dashboard" })).toBeVisible()
+    await expect(page.getByText("E2E Admin")).toBeVisible()
+
+    await page.getByRole("button", { name: "Language: English" }).click()
+    await page.getByRole("menuitemradio", { name: "Spanish" }).click()
+
+    await expect(page).toHaveURL("/")
+    await expect(page.locator("html")).toHaveAttribute("lang", "es")
+    await expect(
+      page.getByRole("heading", { name: "Panel de control" }),
+    ).toBeVisible()
+    await expect(page.getByText("E2E Admin")).toBeVisible()
+    await expect(page.getByText("admin@example.test")).toBeVisible()
+    await expectLocaleCookie(page, "es")
+  })
+})

tests/unit/actions/i18n.actions.test.ts

@@ -0,0 +1,50 @@
+import { beforeEach, describe, expect, it, vi } from "vitest"
+
+import { LOCALE_COOKIE_MAX_AGE_SECONDS } from "@/i18n/locales"
+
+const headersMocks = vi.hoisted(() => ({
+  cookieSet: vi.fn(),
+  cookies: vi.fn(),
+}))
+
+vi.mock("next/headers", () => ({
+  cookies: headersMocks.cookies,
+}))
+
+import { setLocaleAction } from "@/actions/i18n.actions"
+
+describe("setLocaleAction", () => {
+  beforeEach(() => {
+    headersMocks.cookieSet.mockReset()
+    headersMocks.cookies.mockReset()
+    headersMocks.cookies.mockResolvedValue({
+      set: headersMocks.cookieSet,
+    })
+  })
+
+  it("writes a validated supported locale to the locale cookie", async () => {
+    const result = await setLocaleAction("es")
+
+    expect(result).toEqual({ success: true, locale: "es" })
+    expect(headersMocks.cookies).toHaveBeenCalledOnce()
+    expect(headersMocks.cookieSet).toHaveBeenCalledWith(
+      "stock-manager-locale",
+      "es",
+      {
+        path: "/",
+        sameSite: "lax",
+        maxAge: LOCALE_COOKIE_MAX_AGE_SECONDS,
+        httpOnly: true,
+        secure: false,
+      },
+    )
+  })
+
+  it("rejects unsupported locales without writing a cookie", async () => {
+    const result = await setLocaleAction("fr")
+
+    expect(result).toEqual({ success: false, error: "UNSUPPORTED_LOCALE" })
+    expect(headersMocks.cookies).not.toHaveBeenCalled()
+    expect(headersMocks.cookieSet).not.toHaveBeenCalled()
+  })
+})

tests/unit/i18n/dictionaries.test.ts

@@ -26,6 +26,24 @@ describe("i18n dictionaries", () => {
     })
   })
 
+  it("provides localized language switcher copy for English and Spanish", () => {
+    expect(getDictionary("en").common.languageSwitcher).toEqual({
+      label: "Language",
+      options: {
+        en: "English",
+        es: "Spanish",
+      },
+    })
+
+    expect(getDictionary("es").common.languageSwitcher).toEqual({
+      label: "Idioma",
+      options: {
+        en: "Inglés",
+        es: "Español",
+      },
+    })
+  })
+
   it("keeps dashboard home dictionary keys aligned across locales", () => {
     expect(getDictionary("en").dashboardHome).toEqual({
       heading: "Dashboard",

tests/unit/i18n/locales.test.ts

@@ -4,6 +4,7 @@ import {
   DEFAULT_LOCALE_ENV_VAR,
   FALLBACK_LOCALE,
   isLocale,
+  LOCALE_COOKIE_MAX_AGE_SECONDS,
   LOCALE_COOKIE_NAME,
   resolveDefaultLocale,
   resolveLocale,
@@ -16,6 +17,7 @@ describe("i18n locales", () => {
     expect(FALLBACK_LOCALE).toBe("en")
     expect(DEFAULT_LOCALE_ENV_VAR).toBe("STOCK_MANAGER_DEFAULT_LOCALE")
     expect(LOCALE_COOKIE_NAME).toBe("stock-manager-locale")
+    expect(LOCALE_COOKIE_MAX_AGE_SECONDS).toBe(60 * 60 * 24 * 365)
   })
 
   it("accepts only exact supported locale codes", () => {