feat: add internationalization message platform

tests/unit/i18n-contract.test.ts

@@ -0,0 +1,115 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  catalogKeys,
+  FORMAT_FALLBACK,
+  formatDate,
+  formatList,
+  formatMessage,
+  formatNumber,
+  formatRelativeTime,
+  localeDirection,
+  messagePlaceholders,
+  normalizeLocale,
+  resolveMessage,
+  selectMessage,
+  selectPlural,
+} from "../../src/presentation/i18n/index.js";
+import {
+  EN_MESSAGES,
+  KO_MESSAGES,
+  MESSAGE_CATALOGS,
+} from "../../src/presentation/i18n/catalog.js";
+
+describe("internationalization message contract", () => {
+  it("keeps every registered catalog and interpolation contract in parity", () => {
+    expect(catalogKeys("en-US")).toEqual(catalogKeys("ko-KR"));
+    expect(catalogKeys("ar-EG")).toEqual(catalogKeys("ko-KR"));
+
+    for (const key of catalogKeys("ko-KR")) {
+      expect(messagePlaceholders(EN_MESSAGES[key])).toEqual(
+        messagePlaceholders(KO_MESSAGES[key]),
+      );
+    }
+    expect(MESSAGE_CATALOGS["ar-EG"]).toBe(EN_MESSAGES);
+  });
+
+  it("formats typed parameters and expands the pseudo locale", () => {
+    expect(
+      formatMessage("en-US", "route.documentTitle", {
+        title: "Settings",
+        appName: "Skeleton",
+      }),
+    ).toBe("Settings · Skeleton");
+    const pseudo = formatMessage("en-XA", "action.closeNamed", {
+      title: "Account",
+    });
+    expect(pseudo).toMatch(/^［.+ ···］$/);
+    expect(pseudo.length).toBeGreaterThan("Close Account".length);
+  });
+
+  it("falls back safely for unknown locale, key and missing interpolation", () => {
+    expect(normalizeLocale("fr-FR")).toBe("ko-KR");
+    expect(resolveMessage("fr-FR", "action.retry")).toBe("다시 시도");
+    expect(resolveMessage("en-US", "action.login")).toBe("Sign in");
+    expect(resolveMessage("en-US", "backend.stack.trace")).toBe(
+      "요청한 문구를 표시할 수 없습니다.",
+    );
+    expect(resolveMessage("en-US", "action.closeNamed")).toBe(
+      "요청한 문구를 표시할 수 없습니다.",
+    );
+    expect(resolveMessage("en-US", "backend.stack.trace")).not.toContain(
+      "backend.stack.trace",
+    );
+  });
+
+  it("defines direction without inferring it in components", () => {
+    expect(localeDirection("ko-KR")).toBe("ltr");
+    expect(localeDirection("en-XA")).toBe("ltr");
+    expect(localeDirection("ar-EG")).toBe("rtl");
+  });
+});
+
+describe("internationalization formatter contract", () => {
+  const instant = new Date("2026-07-26T23:30:00.000Z");
+
+  it("uses an explicit UTC default and deterministic locale formatting", () => {
+    expect(formatDate("en-US", instant, { dateStyle: "short" })).toBe(
+      "7/26/26",
+    );
+    expect(
+      formatDate("en-US", instant, {
+        dateStyle: "short",
+        timeZone: "Asia/Seoul",
+      }),
+    ).toBe("7/27/26");
+    expect(formatNumber("en-US", 1234.5)).toBe("1,234.5");
+    expect(formatRelativeTime("en-US", -1, "day")).toBe("yesterday");
+    expect(formatList("en-US", ["one", "two", "three"])).toBe(
+      "one, two, and three",
+    );
+    expect(selectPlural("en-US", 1, { one: "item", other: "items" })).toBe(
+      "item",
+    );
+    expect(
+      selectPlural("en-US", 3, { one: "item", other: "items" }),
+    ).toBe("items");
+    expect(
+      selectMessage("pending", {
+        ready: "Ready",
+        pending: "Pending",
+        other: "Unknown",
+      }),
+    ).toBe("Pending");
+  });
+
+  it("does not throw or expose invalid formatter input", () => {
+    expect(formatDate("en-US", Number.NaN)).toBe(FORMAT_FALLBACK);
+    expect(formatNumber("en-US", Number.POSITIVE_INFINITY)).toBe(
+      FORMAT_FALLBACK,
+    );
+    expect(
+      formatDate("en-US", instant, { timeZone: "not/a-time-zone" }),
+    ).toBe(FORMAT_FALLBACK);
+  });
+});