Migrate i18next to Lingui

Migration Checklist

- [ ] Step 1: Install Lingui packages
- [ ] Step 2: Create lingui.config.js
- [ ] Step 3: Set up build tooling (Babel/SWC/Vite)
- [ ] Step 4: Replace i18n initialization
- [ ] Step 5: Migrate React components (useTranslation → useLingui, Trans → Trans)
- [ ] Step 6: Migrate JS/TS strings (t() → t``)
- [ ] Step 7: Migrate plurals
- [ ] Step 8: Migrate namespaces
- [ ] Step 9: Convert existing translation catalogs
- [ ] Step 10: Run lingui extract && lingui compile
- [ ] Step 11: Remove i18next packages

Step 1: Install Lingui

# Core (always required)
npm install @lingui/core @lingui/react

# CLI (dev)
npm install --save-dev @lingui/cli

# Macro support - pick one based on build tool:
# Babel
npm install --save-dev babel-plugin-macros
# SWC
npm install --save-dev @lingui/swc-plugin
# Vite
npm install --save-dev @lingui/vite-plugin

Step 2: Create lingui.config.js

import { defineConfig } from "@lingui/cli";

export default defineConfig({
  sourceLocale: "en",
  locales: ["en", "de", "fr"], // match your existing locales
  catalogs: [
    {
      path: "<rootDir>/src/locales/{locale}/messages",
      include: ["src"],
    },
  ],
});

Step 3: Configure Build Tooling

Vite (vite.config.ts):

import { lingui } from "@lingui/vite-plugin";
import react from "@vitejs/plugin-react";

export default { plugins: [react(), lingui()] };

Babel (.babelrc or babel.config.js):

{ "plugins": ["macros"] }

Next.js with SWC (next.config.js):

module.exports = {
  experimental: {
    swcPlugins: [["@lingui/swc-plugin", {}]],
  },
};

Step 4: Replace i18n Initialization

Before (i18next):

import i18next from "i18next";
import { initReactI18next } from "react-i18next";

i18next.use(initReactI18next).init({
  lng: "en",
  resources: { en: { translation: { key: "Hello world" } } },
});

After (Lingui):

import { i18n } from "@lingui/core";
import { I18nProvider } from "@lingui/react";
import { messages } from "./locales/en/messages";

i18n.load("en", messages);
i18n.activate("en");

function App() {
  return <I18nProvider i18n={i18n}>{/* app */}</I18nProvider>;
}

Dynamic locale switching:

async function changeLocale(locale) {
  const { messages } = await import(`./locales/${locale}/messages`);
  i18n.load(locale, messages);
  i18n.activate(locale);
}

Step 5: Migrate React Components

useTranslation → useLingui

Before:

import { useTranslation } from "react-i18next";

function MyComponent() {
  const { t } = useTranslation();
  return <p>{t("greeting")}</p>;
}

After:

import { useLingui } from "@lingui/react/macro";

function MyComponent() {
  const { t } = useLingui();
  return <p>{t`Hello World`}</p>;
}

Trans component

Before:

import { Trans } from "react-i18next";

<Trans i18nKey="welcome">Hello World!</Trans>

After:

import { Trans } from "@lingui/react/macro";

<Trans>Hello World!</Trans>

For explicit IDs (when preserving i18next keys):

<Trans id="welcome">Hello World!</Trans>

Trans with interpolation

Before:

<Trans i18nKey="greeting" values={{ name }}>Hello {{ name }}!</Trans>

After:

<Trans>Hello {name}!</Trans>

Step 6: Migrate JS/TS Strings

Simple strings

i18next	Lingui
t('key')	t\Message text``
t('key', { name })	t\Hello ${name}``
t('key', { defaultValue: 'Hi' })	t\Hi``

Before:

import i18next from "i18next";

const msg = i18next.t("greeting", { name: "Tom" });

After:

import { useLingui } from "@lingui/react/macro";

function MyComponent() {
  const { t } = useLingui();
  const msg = t`Hello ${name}`;
}

In vanilla JS (outside components):

import { t } from "@lingui/core/macro";

const msg = t`Hello ${name}`;

Preserving explicit keys from i18next

If you want to keep the i18next message IDs:

import { t } from "@lingui/core/macro";

// i18next: t('navigation.home')
const msg = t({ id: "navigation.home", message: "Home" });

Module-level / lazy strings

Before (i18next):

const LABELS = {
  active: "Active",
  inactive: "Inactive",
};
// translated at render time

After (Lingui):

import { msg } from "@lingui/core/macro";
import { useLingui } from "@lingui/react";

const LABELS = {
  active: msg`Active`,
  inactive: msg`Inactive`,
};

function StatusDisplay({ status }) {
  const { _ } = useLingui();
  return <div>{_(LABELS[status])}</div>;
}

Step 7: Migrate Plurals

i18next uses separate keys per plural form. Lingui uses ICU MessageFormat in a single message.

Before (i18next):

{
  "item_one": "{{count}} item",
  "item_other": "{{count}} items"
}

t("item", { count });

After (Lingui) - JSX:

import { Plural } from "@lingui/react/macro";

<Plural value={count} one="# item" other="# items" />

After (Lingui) - JS strings:

import { plural } from "@lingui/core/macro";

const msg = plural(count, {
  one: "# item",
  other: "# items",
});

Exact matches (i18next _0):

<Plural
  value={count}
  _0="No items"
  one="# item"
  other="# items"
/>

Step 8: Migrate Namespaces

i18next namespaces (useTranslation('common')) map to Lingui catalog paths. Two approaches:

Option A - Single catalog (simplest): Remove namespaces and use one unified catalog. Update lingui.config.js to include all source directories.

Option B - Multiple catalogs (preserves namespace separation):

// lingui.config.js
catalogs: [
  {
    path: "<rootDir>/src/locales/{locale}/common",
    include: ["src/components/common"],
  },
  {
    path: "<rootDir>/src/locales/{locale}/auth",
    include: ["src/components/auth"],
  },
]

With multiple catalogs, load them all at startup:

import { messages as commonMessages } from "./locales/en/common";
import { messages as authMessages } from "./locales/en/auth";

i18n.load("en", { ...commonMessages, ...authMessages });
i18n.activate("en");

Step 9: Convert Existing Translation Catalogs

See catalog-conversion.md for and patterns.

Key concept: i18next uses JSON with dotted keys; Lingui uses .po files with the message as the ID (or an explicit ID you provide).

Step 10: Build & Verify

npx lingui extract    # extracts all messages → .po files
npx lingui compile    # compiles .po → .js message catalogs

Add to package.json:

{
  "scripts": {
    "i18n:extract": "lingui extract",
    "i18n:compile": "lingui compile"
  }
}

For TypeScript:

npx lingui compile --typescript

Step 11: Remove i18next

npm uninstall i18next react-i18next

Common Patterns Reference

Date/Number Formatting

Before (i18next):

t("intlDateTime", { val: new Date() });

After (Lingui):

import { useLingui } from "@lingui/react/macro";

function Component() {
  const { i18n } = useLingui();
  return <span>{new Intl.DateTimeFormat(i18n.locale).format(date)}</span>;
}

Context (disambiguation)

Before (i18next):

t("right", { context: "direction" });

After (Lingui):

<Trans context="direction">right</Trans>
// or
t({ message: "right", context: "direction" });

Gender / Select

Before (i18next):

{ "pronoun_male": "He", "pronoun_female": "She", "pronoun_other": "They" }

After (Lingui):

import { select } from "@lingui/core/macro";

const pronoun = select(gender, {
  male: "He",
  female: "She",
  other: "They",
});

Pitfalls

• **Don't call t\...`at module level.** Usemsg`...`instead and translate with_(descriptor)` at render time.
• After locale change, call both i18n.load(locale, messages) and i18n.activate(locale).
• Wrap the entire app in <I18nProvider i18n={i18n}> before any component uses useLingui or Trans.
• Always run lingui compile before building for production; the app imports compiled .js catalogs, not .po files.
• Generated IDs change if message text changes. If stability matters, use explicit id props.

Additional Resources

• Detailed migration patterns
• Catalog conversion guide
• Lingui docs
• Lingui vs i18next comparison