src/i18n/config.test.ts

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+import { describe, expect, it } from "vitest";
+import { createI18nInstance } from "./config.js";
+
+describe("createI18nInstance", () => {
+  it("creates an initialized i18next instance for the given locale", async () => {
+    const i18n = await createI18nInstance({ locale: "ru" });
+    expect(i18n.language).toBe("ru");
+    expect(i18n.isInitialized).toBe(true);
+  });
+
+  it("can translate a known key from the Russian locale", async () => {
+    const i18n = await createI18nInstance({ locale: "ru" });
+    const value = i18n.t("BOARD.DEPARTURE");
+    // The Russian file has "BOARD.DEPARTURE" = "Вылет"
+    expect(value).toBe("Вылет");
+  });
+
+  it("can translate a known key from the English locale", async () => {
+    const i18n = await createI18nInstance({ locale: "en" });
+    const value = i18n.t("BOARD.DEPARTURE");
+    expect(value).toBe("Departure");
+  });
+
+  it("uses initialResources instead of loading from filesystem when provided", async () => {
+    const i18n = await createI18nInstance({
+      locale: "ru",
+      initialResources: {
+        common: { TEST_KEY: "Тестовое значение" },
+      },
+    });
+    expect(i18n.t("TEST_KEY")).toBe("Тестовое значение");
+  });
+
+  it("returns the key path if a key is missing (no fallback to other locale)", async () => {
+    const i18n = await createI18nInstance({ locale: "ru" });
+    expect(i18n.t("NONEXISTENT.KEY")).toBe("NONEXISTENT.KEY");
+  });
+
+  it("each call returns a fresh instance (request-scoped)", async () => {
+    const a = await createI18nInstance({ locale: "ru" });
+    const b = await createI18nInstance({ locale: "en" });
+    expect(a).not.toBe(b);
+    expect(a.language).toBe("ru");
+    expect(b.language).toBe("en");
+  });
+});

src/i18n/config.ts

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+import i18next from "i18next";
+import ICU from "i18next-icu";
+import resourcesToBackend from "i18next-resources-to-backend";
+import type { Language } from "./resolver.js";
+
+export async function createI18nInstance(options: {
+  locale: Language;
+  initialResources?: Record<string, Record<string, unknown>>;
+}): Promise<typeof i18next> {
+  const instance = i18next.createInstance();
+
+  instance.use(ICU);
+
+  // If no pre-loaded resources, load from the locale JSON files on the filesystem.
+  if (!options.initialResources) {
+    instance.use(
+      resourcesToBackend(
+        (language: string, namespace: string) =>
+          import(`./locales/${language}/${namespace}.json`),
+      ),
+    );
+  }
+
+  await instance.init({
+    lng: options.locale,
+    ns: ["common"],
+    defaultNS: "common",
+    fallbackLng: false,
+    interpolation: {
+      escapeValue: false,
+    },
+    keySeparator: ".",
+    ...(options.initialResources
+      ? {
+          resources: {
+            [options.locale]: options.initialResources,
+          },
+        }
+      : {}),
+  });
+
+  return instance;
+}