Lovable I18N

Lovable i18n with Lingui

You are the Lovable agent working on one project. You cannot run terminal commands — no npm, no lingui CLI, no git. You can edit files, add npm dependencies, and read preview build errors and console logs. Everything below is designed around that.

Three consequences of that design, worth internalizing before you start:

1. You never run lingui compile. @lingui/vite-plugin compiles .po catalogs at build time, when the app dynamically imports them. The preview build is the compiler.
2. You never run lingui extract. A GitHub Actions workflow (Phase 5) runs extraction in CI after Lovable syncs commits to GitHub. Until that lands, you maintain .po files by hand, following the catalog maintenance protocol later in this skill.
3. You scaffold catalogs yourself. Since nothing can run the CLI here, you create the messages.po files directly, with valid PO headers (step A7).

The library is Lingui v6, with one PO catalog per locale at src/locales/{locale}/messages.po.

Because the whole pipeline — the catalog-sync workflow (Phase 5) and the Globalize.now hand-off (Phase 6) — runs through GitHub, this skill confirms GitHub is connected before doing anything, and stops until it is. See Prerequisite: GitHub connection below.

One thing the terminal limit does not block: connecting Globalize. Once the user adds the Globalize MCP connector (Phase 6), you create the project and link the repo yourself, right here in the chat — no leaving Lovable. A paste-ready fallback covers anyone who'd rather not add the connector.

Phases

Work the phases in order. Detect and Ask are quick; Setup is the bulk of the work. Before Phase 1, confirm GitHub is connected (Prerequisite below) — the skill stops until it is. Phase 6 (Connect) runs by default unless the user opts out.

Prerequisite: GitHub connection

Before any i18n work — before even detecting the stack — confirm this Lovable project is connected to GitHub. This is a hard gate. Everything this skill ultimately delivers runs through GitHub: the catalog-sync workflow (Phase 5) and the Globalize.now translation hand-off (Phase 6) both operate on the GitHub repository, which lives outside Lovable's sandbox. Globalize reads your message catalogs from GitHub and delivers translations back through it — there is no supported way to send your text out for translation or get it back without a connected repo.

You know whether GitHub is connected from your Lovable project context — you don't need to read a file or run any command to tell. (If you genuinely can't determine it from your context, ask the user to confirm before continuing.) Do not infer connection from files in the tree: .gitignore, package.json, and a .git folder ship in every Lovable scaffold whether or not GitHub is connected, so they're false positives.

• If GitHub is connected → say so in one line and continue to Phase 1.

• If GitHub is not connected → stop here. Don't detect the stack, don't install anything, don't edit files. Tell the user:

  To translate this app I need it connected to GitHub first. The translation platform (Globalize.now) and the catalog-sync workflow both read and write your message catalogs through your GitHub repo — without it, there's no way to send your text out for translation or get it back.

  Connect it with the GitHub button at the top-right of the editor (or Settings → GitHub): authorize the Lovable GitHub App and create or link a repository. Once it's connected, tell me to continue and I'll set up translations.

  Then wait. Resume at Phase 1 once the user confirms GitHub is connected.

Phase 1: Detect & Ask

1.1 Detect

Read package.json and decide which stack this project is. First match wins.

Path B — TanStack Start (SSR). Authoritative signal: @tanstack/react-start in dependencies. Corroborating signals (any of these confirm, none is required once the dependency is present):

• src/routes/__root.tsx exists
• src/router.tsx exporting a getRouter() function
• @lovable.dev/vite-tanstack-config wrapping the config in vite.config.ts
• wrangler.jsonc at the project root (Cloudflare Workers deploy target)

→ Follow Phase 2B: Setup — TanStack Start (SSR).

Path A — Vite SPA (legacy stack). vite in devDependencies and no @tanstack/react-start. Corroborating signals:

• @vitejs/plugin-react-swc in devDependencies
• react-router-dom in dependencies and a <Routes> block in src/App.tsx
• components.json at the project root (shadcn/ui)
• lovable-tagger in devDependencies
• root index.html and src/main.tsx

→ Follow Phase 2A: Setup — Vite SPA.

If neither matches, tell the user what you found and that this skill covers Lovable's two project stacks only — don't guess your way into a setup.

Escape hatches

Check these before starting setup. They change or stop the plan.

Babel instead of SWC (Path A). If vite.config.ts uses @vitejs/plugin-react (no -swc suffix), the SWC macro plugin won't work. Tell the user, then substitute the Babel plugin:

This project uses the Babel-based React plugin instead of SWC. Same result, slightly different wiring: I'll use @lingui/babel-plugin-lingui-macro instead of the SWC plugin.

Add @lingui/babel-plugin-lingui-macro@^6 as a dev dependency (instead of @lingui/swc-plugin), and in A2 configure the React plugin as:

react({
  babel: {
    plugins: ['@lingui/babel-plugin-lingui-macro'],
  },
})

Everything else in Phase 2A is identical.

Existing i18n library. If react-i18next, i18next, react-intl, next-translate, or any other i18n library is already in dependencies: STOP and ask the user. Two options: keep the existing library (this skill doesn't apply — its setup, catalogs, and CI are Lingui-specific), or remove the existing library and its usages first, then re-run this skill. Never migrate or rip out an i18n library silently.

Existing Lingui config. If a lingui.config.ts (or .js) already exists, run in additive mode: verify the config matches the shape in A3 (PO formatter, src/locales/{locale}/messages catalog path), add any locales the user requested that are missing (config + new .po files per A7), confirm the provider is wired (A5) and the vite plugins are present (A2) — or their Phase 2B equivalents on Path B — fix only what's missing, then continue from Phase 3 (make sure the AGENTS.md coding rules are in place) before wrapping strings in Phase 4.

1.2 Ask

Auto-detect before asking — pull defaults from the project so the user mostly confirms:

• Source locale: the <html lang="..."> value in index.html; any existing src/locales/<locale>/ directories; default en.
• Target locales: any language the user already named in their request ("add Spanish" → es is pre-selected); existing locale directories.

Then send one chat message with every question and a sensible default pre-selected. Do not drip-feed questions one at a time. Include question 3 only on Path A (Vite SPA) — on Path B locale handling is cookie-based by default (see B6); URL-prefix routing is only added if the user asks. Shape it like this:

Before I set up translations, a few choices — defaults in bold, just say "go" to accept all:

1. Source language — the language your app is written in. Detected: en (from <html lang>).
2. Target languages — which languages to translate into. You mentioned Spanish (es); common additions: French (fr), German (de), Portuguese (pt), Japanese (ja). Which do you want?
3. Locale in the URL?
   • No URL locale (default) — language is remembered per visitor (saved choice → browser language). URLs stay exactly as they are. Simplest, no link changes.
   • URL prefix (/es/dashboard) — every page exists per language; shareable, SEO-friendly language URLs, but all internal links need a locale prefix.
4. Catalog sync via GitHub Actions — a workflow that extracts new texts into the catalogs whenever code changes. Default: yes. GitHub is already connected (the prerequisite), so this just adds the workflow.
5. Connect Globalize.now — the translation platform that fills in the actual translations via PRs. Default: yes. I'll connect it through the Globalize MCP connector (a quick one-time setup I'll walk you through). Say "skip Globalize" to set it up later.

After the user answers, execute the whole plan without further pauses. Only stop again for blockers: a build error you cannot resolve, or an escape hatch from 1.1. GitHub connection is already settled by the prerequisite, so it's not a mid-flow stop.

Record the answers — SOURCE_LOCALE, the locale list, routing choice, opt-ins — you will substitute them into every snippet below. The snippets use en as source and ['en', 'es', 'fr'] as the locale list; replace with the real choices everywhere.

Narrate structural edits. Before modifying build config or app-entry files — vite.config.ts, src/main.tsx, src/App.tsx, index.html on Path A; vite.config.ts, src/router.tsx, src/routes/__root.tsx, src/start.ts on Path B — state in one chat sentence what will change and why (e.g. "I'm adding the Lingui plugins to vite.config.ts so translations compile at build time"), then make the edit. Don't wait for permission — just narrate.

Phase 2A: Setup — Vite SPA

Nine steps, A1–A9, in order. Then the optional URL-routing variant if the user opted in.

A1. Dependencies

Add these dependencies (do not write install commands — add them to the project's dependencies directly):

Runtime dependencies:

Dev dependencies:

Version pinning caveat: @lingui/swc-plugin must match the swc_core version shipped by @vitejs/plugin-react-swc. If the preview build fails with an AST schema error or a plugin invocation error after this setup, look up the compatible version at https://plugins.swc.rs and pin @lingui/swc-plugin to that exact version (e.g. 5.8.0 — no caret). This exact pin overrides the ^6 default for this one package only.

A2. vite.config.ts

Add two things: the SWC macro plugin inside the existing react() call, and lingui() as a top-level Vite plugin. Preserve everything already there — lovable-tagger's componentTagger(), the @ path alias, server options, conditional plugin logic. Only add, never replace.

A typical Lovable Vite config becomes:

import { defineConfig } from 'vite'
import react from '@vitejs/plugin-react-swc'
import { lingui } from '@lingui/vite-plugin'
import { componentTagger } from 'lovable-tagger'
import path from 'path'

export default defineConfig(({ mode }) => ({
  server: {
    host: '::',
    port: 8080,
  },
  plugins: [
    react({
      plugins: [['@lingui/swc-plugin', {}]],
    }),
    lingui(),
    mode === 'development' && componentTagger(),
  ].filter(Boolean),
  resolve: {
    alias: {
      '@': path.resolve(__dirname, './src'),
    },
  },
}))

The SWC plugin entry must be the tuple shape ['@lingui/swc-plugin', {}] — a string + options object. Passing the plugin name as a bare string silently disables the macro transform: the build succeeds, but <Trans> never resolves and raw macro output leaks into the UI.

A3. lingui.config.ts

Create lingui.config.ts at the project root:

// lingui.config.ts
import type { LinguiConfig } from '@lingui/conf'
import { formatter } from '@lingui/format-po'

const config: LinguiConfig = {
  sourceLocale: 'en',
  locales: ['en', 'es', 'fr'],
  catalogs: [
    {
      path: '<rootDir>/src/locales/{locale}/messages',
      include: ['<rootDir>/src'],
      exclude: ['**/node_modules/**', '**/locales/**'],
    },
  ],
  format: formatter({ lineNumbers: false }),
}

export default config

Substitute sourceLocale and locales from the Ask phase. format: formatter(...) from @lingui/format-po is required — Lingui 6 removed the format: 'po' string form, so a string here fails. lineNumbers: false keeps #: references to file paths only — line numbers would change on almost every edit and generate a CI catalog-sync commit after every push; paths alone are stable. The **/locales/** exclusion keeps the extractor (in CI) from scanning the catalogs themselves.

A4. src/i18n.ts

Create src/i18n.ts — locale constants, detection, activation, persistence:

// src/i18n.ts
import { i18n } from '@lingui/core'
import { detect, fromUrl, fromStorage, fromNavigator } from '@lingui/detect-locale'

// Must match the `locales` array in lingui.config.ts
export const LOCALES: readonly string[] = ['en', 'es', 'fr']
export const SOURCE_LOCALE = 'en'
const RTL_LOCALES = new Set(['ar', 'he', 'fa', 'ur', 'ps', 'sd', 'yi'])

function getDirection(locale: string): 'ltr' | 'rtl' {
  return RTL_LOCALES.has(locale.split('-')[0]) ? 'rtl' : 'ltr'
}

export function detectLocale(): string {
  let detected: string | null
  try {
    detected = detect(fromUrl('lang'), fromStorage('lang'), fromNavigator())
  } catch {
    // localStorage threw (sandboxed iframe / blocked storage) — retry without it
    detected = detect(fromUrl('lang'), fromNavigator())
  }
  if (detected) {
    if (LOCALES.includes(detected)) return detected
    // Regional fallback: es-MX → es
    const base = detected.split('-')[0]
    if (LOCALES.includes(base)) return base
  }
  return SOURCE_LOCALE
}

export async function activateLocale(locale: string) {
  try {
    const { messages } = await import(`./locales/${locale}/messages.po`)
    i18n.loadAndActivate({ locale, messages })
  } catch (e) {
    console.error(`Failed to load "${locale}" catalog, falling back to "${SOURCE_LOCALE}"`, e)
    const { messages } = await import(`./locales/${SOURCE_LOCALE}/messages.po`)
    i18n.loadAndActivate({ locale: SOURCE_LOCALE, messages })
  }
  document.documentElement.lang = i18n.locale
  document.documentElement.dir = getDirection(i18n.locale)
}

export function saveLocale(locale: string) {
  try {
    localStorage.setItem('lang', locale)
  } catch {
    // Storage unavailable — the choice just won't persist
  }
}

export { i18n }

Notes:

• The dynamic import targets the .po file directly (./locales/${locale}/messages.po) — @lingui/vite-plugin compiles it to runtime messages at that moment. No compiled .js/.ts catalogs ever exist in the repo.
• detectLocale() tries sources in order: ?lang= URL parameter → lang key in localStorage → browser language (with regional fallback, es-MX → es) → source locale.
• activateLocale() also keeps <html lang> and <html dir> in sync, so RTL locales (Arabic, Hebrew, Farsi, Urdu…) flip the document direction automatically.
• Call saveLocale() only on an explicit user choice (the language switcher), so the choice persists across visits.
• Why the try/catch around storage: in sandboxed preview iframes and cookie-blocking browsers, touching localStorage throws a SecurityError — unguarded, that happens inside detectLocale() before first render and leaves the app blank.

TypeScript doesn't know what a .po import is, so add a module declaration. Append to src/vite-env.d.ts (it exists in every Lovable Vite project), or create src/po-modules.d.ts if you prefer not to touch it:

declare module '*.po' {
  import type { Messages } from '@lingui/core'
  export const messages: Messages
}

A5. Provider in src/main.tsx

Wrap the app with I18nProvider, and detect + activate the locale before the first render so the UI never flashes untranslated. Preserve the file's existing imports (./index.css, etc.):

// src/main.tsx
import { createRoot } from 'react-dom/client'
import { I18nProvider } from '@lingui/react'
import { i18n, detectLocale, activateLocale } from './i18n'
import App from './App.tsx'
import './index.css'

async function bootstrap() {
  await activateLocale(detectLocale())
  createRoot(document.getElementById('root')!).render(
    <I18nProvider i18n={i18n}>
      <App />
    </I18nProvider>,
  )
}

void bootstrap()

If main.tsx already wraps <App /> in other providers, keep them and put I18nProvider outermost (everything that renders text needs it above them in the tree).

The preview will show a dynamic-import error from this point until step A7 creates the .po catalogs — complete A7 before reading anything into the build output.

A6. index.html

Check the <html lang="..."> value at the project root:

• Set it to the source locale (e.g. <html lang="en">). It's the pre-JavaScript default; activateLocale() takes over at runtime.
• If the existing value disagrees with the chosen source locale, flag it to the user — one of the two is wrong.
• Remove any hardcoded dir attribute. activateLocale() sets dir dynamically; a hardcoded value flashes the wrong direction for RTL locales.

A7. Scaffold the catalogs

Nothing here can run lingui extract, so create the catalog files yourself. For every locale — including the source locale — create src/locales/{locale}/messages.po containing only a valid PO header:

msgid ""
msgstr ""
"Content-Type: text/plain; charset=utf-8\n"
"Language: es\n"
"MIME-Version: 1.0\n"
"Plural-Forms: nplurals=2; plural=(n != 1);\n"

Set Language: to the file's locale. Set Plural-Forms from this list where the locale appears (one line per locale group, expression after the →); omit the line otherwise (Lingui stores plurals as ICU expressions inside messages, so the header is informational):

en, es, de, it, nl, pt → nplurals=2; plural=(n != 1);
fr, pt-BR, tr          → nplurals=2; plural=(n > 1);
ja, zh, ko, th, vi, id → nplurals=1; plural=0;
ru, uk                 → nplurals=3; plural=(n%10==1 && n%100!=11 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2);
pl                     → nplurals=3; plural=(n==1 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2);
ar                     → nplurals=6; plural=(n==0 ? 0 : n==1 ? 1 : n==2 ? 2 : n%100>=3 && n%100<=10 ? 3 : n%100>=11 ? 4 : 5);

A header-only catalog is valid: it compiles to an empty message set, and Lingui falls back to the source text for any missing message. Entries get added by the Wrap phase and by CI extraction later.

A8. Language switcher

Every Lovable project ships shadcn/ui, so build the switcher on the shadcn Select:

// src/components/LanguageSwitcher.tsx
import { useLingui } from '@lingui/react/macro'
import {
  Select,
  SelectContent,
  SelectItem,
  SelectTrigger,
  SelectValue,
} from '@/components/ui/select'
import { LOCALES, activateLocale, saveLocale } from '@/i18n'

export function LanguageSwitcher() {
  const { i18n } = useLingui()
  const displayNames = new Intl.DisplayNames([i18n.locale], { type: 'language' })

  async function switchLocale(locale: string) {
    await activateLocale(locale)
    saveLocale(locale)
  }

  return (
    <Select value={i18n.locale}
      <SelectTrigger className="w-[140px]">
        <SelectValue />
      </SelectTrigger>
      <SelectContent>
        {LOCALES.map((locale) => (
          <SelectItem key={locale} value={locale}>
            {displayNames.of(locale) ?? locale}
          </SelectItem>
        ))}
      </SelectContent>
    </Select>
  )
}

useLingui() subscribes the component to locale changes, so the selected value updates after activateLocale() resolves. Intl.DisplayNames renders each language's name in the current locale ("Spanish" / "espagnol" / "Spanisch") with no hand-maintained name map.

Place the switcher where the user can see it — the app's existing header or navigation component if there is one, otherwise the top-level layout in App.tsx. Match the surrounding styling (Tailwind classes) rather than keeping the bare w-[140px].

A9. Verify via the preview

You can't run a build, but you can read the preview:

1. The preview builds with no errors. If it fails mentioning the SWC plugin, an AST schema, or plugin invocation — that's the version-pinning caveat from A1; pin @lingui/swc-plugin exactly.
2. Switching languages works. Pick a locale in the switcher; document.documentElement.lang updates (visible in the element inspector, or log it), and the choice survives a reload (localStorage).
3. Strings still show source text. Expected — the catalogs are empty until strings are wrapped (Phase 4) and translations arrive (Phase 6). No console errors about failed catalog imports should appear.

Tell the user what to expect at this point: the plumbing is live, the visible text doesn't change yet.

Opt-in: URL locale routing (/:locale prefix)

Only if the user chose the URL-prefix option in the Ask phase. Lovable Vite SPAs use declarative react-router-dom routes in src/App.tsx — wrap the existing <Routes> content in a locale segment.

1. Locale layout. Create a layout route that validates the URL locale and activates it:

// src/components/LocaleLayout.tsx
import { useEffect } from 'react'
import { Navigate, Outlet, useLocation, useParams } from 'react-router-dom'
import { LOCALES, SOURCE_LOCALE, activateLocale } from '@/i18n'

export function LocaleLayout() {
  const { locale } = useParams()
  const { pathname } = useLocation()
  const valid = locale !== undefined && LOCALES.includes(locale)

  useEffect(() => {
    if (valid) void activateLocale(locale!)
  }, [locale, valid])

  if (!valid) {
    // Bare path like /dashboard — re-prefix with the source locale
    return <Navigate to={`/${SOURCE_LOCALE}${pathname}`} replace />
  }
  return <Outlet />
}

2. Route tree. In src/App.tsx, nest the existing routes under /:locale (the old path="/" route becomes index; other paths lose their leading slash) and redirect the bare root:

import { Navigate, Route, Routes } from 'react-router-dom'
import { LocaleLayout } from '@/components/LocaleLayout'
import { detectLocale } from '@/i18n'

// Inside the existing <BrowserRouter>:
<Routes>
  <Route path="/:locale" element={<LocaleLayout />}>
    <Route index element={<Index />} />
    <Route path="dashboard" element={<Dashboard />} />
    {/* ...every other existing route, path without the leading slash... */}
    <Route path="*" element={<NotFound />} />
  </Route>
  <Route path="/" element={<Navigate to={`/${detectLocale()}`} replace />} />
</Routes>

Bare deep links (/dashboard) match /:locale with an invalid param and get re-prefixed by LocaleLayout; the bare root (/) redirects to the visitor's detected locale. Keep the main.tsx bootstrap from A5 unchanged — it sets the initial locale, and LocaleLayout re-activates per URL from then on.

3. Link helper. Internal links need the prefix:

// src/localePath.ts
export function localePath(locale: string, path: string): string {
  const normalized = path.startsWith('/') ? path : `/${path}`
  return `/${locale}${normalized}`
}

import { Link, useParams } from 'react-router-dom'
import { localePath } from '@/localePath'
import { SOURCE_LOCALE } from '@/i18n'

function Navigation() {
  const { locale } = useParams()
  const currentLocale = locale ?? SOURCE_LOCALE

  return (
    <nav>
      <Link to={localePath(currentLocale, '/')}>Home</Link>
      <Link to={localePath(currentLocale, '/dashboard')}>Dashboard</Link>
    </nav>
  )
}

Update every existing internal <Link to="/...">, <a href="/..."> (internal paths), and navigate('/...') call to go through localePath(). Navigation components (header, sidebar, footer) first — they're on every page.

4. Switcher navigates instead of activating. With the locale in the URL, switching language means navigating to the same page under the new prefix — LocaleLayout handles activation:

// src/components/LanguageSwitcher.tsx (routing variant)
import { useLingui } from '@lingui/react/macro'
import { useLocation, useNavigate, useParams } from 'react-router-dom'
import {
  Select,
  SelectContent,
  SelectItem,
  SelectTrigger,
  SelectValue,
} from '@/components/ui/select'
import { LOCALES, saveLocale } from '@/i18n'

export function LanguageSwitcher() {
  const { i18n } = useLingui()
  const { locale } = useParams()
  const { pathname } = useLocation()
  const navigate = useNavigate()
  const displayNames = new Intl.DisplayNames([i18n.locale], { type: 'language' })

  function switchLocale(next: string) {
    saveLocale(next)
    const basePath = locale ? pathname.slice(locale.length + 1) : pathname
    navigate(`/${next}${basePath || '/'}`)
  }

  return (
    <Select value={i18n.locale}
      <SelectTrigger className="w-[140px]">
        <SelectValue />
      </SelectTrigger>
      <SelectContent>
        {LOCALES.map((loc) => (
          <SelectItem key={loc} value={loc}>
            {displayNames.of(loc) ?? loc}
          </SelectItem>
        ))}
      </SelectContent>
    </Select>
  )
}

Verify in the preview: / redirects to /<locale>, deep links re-prefix, switching languages keeps you on the same page under the new prefix.

Phase 2B: Setup — TanStack Start (SSR)

Eight steps, B1–B8, in order. The defining constraint of this path: the app renders on the server (Cloudflare Workers), so i18n state must be per-request. A module-level i18n singleton activated per request would leak locales across concurrent users — request 2 activating fr mid-render of request 1 sends French HTML to an English visitor. Every server-side snippet below creates a fresh setupI18n() instance per request instead.

Locale selection on this path is cookie-based: a locale cookie carries the visitor's choice, the accept-language header is the fallback, and the server picks the locale before rendering — so the first paint is already in the right language, with no client-side flash.

B1. Dependencies

Add these dependencies (do not write install commands — add them to the project's dependencies directly):

Runtime dependencies:

Do not add @lingui/detect-locale on this path — it reads window and localStorage and throws on the server. Locale detection happens from the request (cookie + accept-language) in B4.

Dev dependencies:

Note the macro plugin difference from Path A: Lovable's TanStack Start template ships the Babel React plugin, so this path always uses @lingui/babel-plugin-lingui-macro, never @lingui/swc-plugin.

B2. vite.config.ts

Lovable's Start template wraps the entire Vite config in @lovable.dev/vite-tanstack-config — it bundles tanstackStart(), viteReact(), Tailwind, the Cloudflare plugin, lovable-tagger, env injection, and the @ alias internally. Do not add any of those plugins manually — duplicates break the build (the config's own comment warns about this). The wrapper exposes exactly the two options Lingui needs: plugins for extra Vite plugins and react for options forwarded to viteReact():

// vite.config.ts
import { defineConfig } from '@lovable.dev/vite-tanstack-config'
import { lingui } from '@lingui/vite-plugin'

export default defineConfig({
  plugins: [lingui()],
  react: { babel: { plugins: ['@lingui/babel-plugin-lingui-macro'] } },
})

Preserve any options already passed to defineConfig — merge, don't replace.

Fallback — Start project without the Lovable wrapper. If vite.config.ts uses plain Vite defineConfig with explicit plugins, the order matters: lingui(), then tanstackStart(), then the React plugin (it must come after Start's):

import { defineConfig } from 'vite'
import { tanstackStart } from '@tanstack/react-start/plugin/vite'
import viteReact from '@vitejs/plugin-react'
import { lingui } from '@lingui/vite-plugin'

export default defineConfig({
  plugins: [
    lingui(),
    tanstackStart(),
    viteReact({ babel: { plugins: ['@lingui/babel-plugin-lingui-macro'] } }),
  ],
})

B3. lingui.config.ts

Identical to Path A — create the file exactly as in A3 (PO formatter, src/locales/{locale}/messages catalog path, locales from the Ask phase).

B4. Per-request i18n: shared module, locale resolution, middleware, router

This is the load-bearing step — six files. Pick these paths and keep them consistent everywhere.

1. src/i18n.ts — isomorphic (safe to import on server and client). No document, no localStorage:

// src/i18n.ts
import type { Messages } from '@lingui/core'

// Must match the `locales` array in lingui.config.ts
export const LOCALES: readonly string[] = ['en', 'es', 'fr']
export const SOURCE_LOCALE = 'en'
const RTL_LOCALES = new Set(['ar', 'he', 'fa', 'ur', 'ps', 'sd', 'yi'])

export function getDirection(locale: string): 'ltr' | 'rtl' {
  return RTL_LOCALES.has(locale.split('-')[0]) ? 'rtl' : 'ltr'
}

export async function loadCatalog(locale: string): Promise<Messages> {
  try {
    const { messages } = await import(`./locales/${locale}/messages.po`)
    return messages
  } catch (e) {
    console.error(`Failed to load "${locale}" catalog, falling back to "${SOURCE_LOCALE}"`, e)
    const { messages } = await import(`./locales/${SOURCE_LOCALE}/messages.po`)
    return messages
  }
}

The dynamic import targets the .po file directly — @lingui/vite-plugin compiles it into the bundle at build time, so catalog loading works on Cloudflare Workers with no runtime filesystem. Add the same *.po module declaration from A4 (in src/vite-env.d.ts or src/po-modules.d.ts).

2. src/modules/lingui/i18n.server.ts — server-only locale resolution from the request. Plain string parsing, no extra dependencies:

// src/modules/lingui/i18n.server.ts
import { LOCALES, SOURCE_LOCALE } from '@/i18n'

function matchLocale(candidate: string): string | null {
  if (LOCALES.includes(candidate)) return candidate
  // Regional fallback: es-MX → es
  const base = candidate.split('-')[0]
  if (LOCALES.includes(base)) return base
  return null
}

export function getLocaleFromRequest(request: Request): string {
  // 1. Explicit choice — `locale` cookie set by the language switcher (B7)
  const cookieHeader = request.headers.get('cookie')
  if (cookieHeader) {
    for (const part of cookieHeader.split(';')) {
      const [name, ...rest] = part.trim().split('=')
      if (name === 'locale') {
        // No decodeURIComponent: locale tags are plain ASCII, and a malformed
        // or foreign cookie value simply won't match LOCALES.
        const matched = matchLocale(rest.join('='))
        if (matched) return matched
      }
    }
  }

  // 2. Browser preference — accept-language header, entries in preference order
  const acceptLanguage = request.headers.get('accept-language')
  if (acceptLanguage) {
    for (const part of acceptLanguage.split(',')) {
      const candidate = part.split(';')[0].trim() // strip ;q=... weights
      if (!candidate || candidate === '*') continue
      const matched = matchLocale(candidate)
      if (matched) return matched
    }
  }

  // 3. Default
  return SOURCE_LOCALE
}

3. src/modules/lingui/lingui-middleware.ts — request middleware that builds the per-request i18n instance and passes it down through the start context:

// src/modules/lingui/lingui-middleware.ts
import { setupI18n } from '@lingui/core'
import { createMiddleware } from '@tanstack/react-start'
import { loadCatalog } from '@/i18n'
import { getLocaleFromRequest } from './i18n.server'

export const linguiMiddleware = createMiddleware({ type: 'request' }).server(
  async ({ request, next }) => {
    const locale = getLocaleFromRequest(request)
    const i18n = setupI18n() // fresh instance per request — never a module-level singleton
    i18n.load(locale, await loadCatalog(locale))
    i18n.activate(locale)
    return next({ context: { locale, i18n } })
  },
)

4. src/start.ts — register the middleware. If the file already exists (the template sometimes ships it), add linguiMiddleware to the existing requestMiddleware array; if not, create it:

// src/start.ts
import { createStart } from '@tanstack/react-start'
import { linguiMiddleware } from './modules/lingui/lingui-middleware'

export const startInstance = createStart(() => ({
  requestMiddleware: [linguiMiddleware],
}))

5. src/router.tsx — getRouter() is called once per request on the server (the template already creates a fresh QueryClient per call for the same reason). Pull the per-request i18n from the start context, wrap the app in I18nProvider, and wire dehydrate/hydrate so the client first paint matches the server exactly:

// src/router.tsx
import { setupI18n, type I18n } from '@lingui/core'
import { I18nProvider } from '@lingui/react'
import { createRouter } from '@tanstack/react-router'
import { getGlobalStartContext } from '@tanstack/react-start'
import { SOURCE_LOCALE } from './i18n'
import { routeTree } from './routeTree.gen'

export function getRouter() {
  // Server: the lingui middleware put a per-request i18n instance in the start
  // context. Client: no middleware ran — create a fresh instance; hydrate()
  // below fills it from the server's dehydrated state before first render.
  const i18n: I18n =
    getGlobalStartContext()?.i18n ??
    setupI18n({ locale: SOURCE_LOCALE, messages: { [SOURCE_LOCALE]: {} } })

  return createRouter({
    routeTree,
    context: { i18n }, // merge into the existing context object (queryClient, ...)
    Wrap: ({ children }) => <I18nProvider i18n={i18n}>{children}</I18nProvider>,
    dehydrate: () => ({
      locale: i18n.locale,
      messages: i18n.messages, // catalog for the active locale
    }),
    hydrate: (dehydrated) => {
      i18n.loadAndActivate({ locale: dehydrated.locale, messages: dehydrated.messages })
    },
  })
}

Keep everything already in getRouter() — the QueryClient, existing context entries, defaultPreload, etc. Only add the i18n pieces.

API drift warning: getGlobalStartContext has been moving between @tanstack/react-start versions. If the import doesn't exist in the installed version, check how the installed @tanstack/react-start exposes the request-scoped start context (the export name and location have changed across releases) and adapt this one line — the rest of the pattern is unchanged. The ?.i18n access also requires the middleware's context type to be registered with Start's type system (the createStart registration in step 4); if TypeScript complains about the property, check how the installed version registers request-middleware context types.

6. src/routes/__root.tsx — extend the route context type with i18n and replace the template's hardcoded <html lang="en">. The template's root route uses createRootRouteWithContext with a shellComponent (RootDocument); Wrap from step 5 renders above the shell, so useLingui() works here:

// src/routes/__root.tsx (i18n-relevant parts — keep everything else in the file)
import type { I18n } from '@lingui/core'
import { useLingui } from '@lingui/react/macro'
import { createRootRouteWithContext, HeadContent, Scripts } from '@tanstack/react-router'
import { getDirection } from '@/i18n'

// Add `i18n` to the EXISTING context type — e.g. if the template already has
// { queryClient: QueryClient }, it becomes { queryClient: QueryClient; i18n: I18n }
export const Route = createRootRouteWithContext<{ i18n: I18n }>()({
  shellComponent: RootDocument,
})

function RootDocument({ children }: { children: React.ReactNode }) {
  const { i18n } = useLingui()
  return (
    <html lang={i18n.locale} dir={getDirection(i18n.locale)}>
      <head>
        <HeadContent />
      </head>
      <body>
        {children}
        <Scripts />
      </body>
    </html>
  )
}

Because the server resolved the locale before rendering, the HTML arrives already translated with the correct lang and dir — no client-side correction, no flash.

Do not localize head() / meta titles yet. Putting translated strings in a route's head() triggers a known TanStack Router hydration bug (issue #4279) that duplicates head content. Leave route titles and meta descriptions in the source language for now; localize body content only.

B5. Scaffold the catalogs

Same as Path A — create src/locales/{locale}/messages.po for every locale exactly as in A7 (valid PO header, Language: and Plural-Forms per locale, source locale included).

B6. Routing: cookie-based by default

The default flow keeps URLs untouched — the locale lives in the cookie, and every URL serves the visitor's language. No route changes, no link rewriting.

A URL-prefix variant (/es/dashboard via an src/routes/$locale/ layout route that validates the param and activates that locale's catalog) exists and matters more under SSR than in a SPA — per-language URLs are crawlable and indexable. It's out of scope for the default flow; mention it to the user as a future option if they ask about SEO for translated pages.

B7. Language switcher

Two files. The switcher calls a server function that sets the locale cookie, then does a full reload so the server re-renders everything under the new cookie — simple and correct, since i18n state is per-request.

Critical file-split rule: a file that defines createServerFn must not also use Lingui macros — the macro transform and the server-function extraction conflict and break the build. Keep the server function in its own macro-free file. Inside any server-function handler that needs translated text, use i18n._(msg`...`) with the request's i18n instance, which arrives via the request-middleware context — e.g. handler(async ({ context }) => context.i18n._(msg`Saved`)) — never bare t macros.

1. src/modules/lingui/locale-fn.ts — the server function (no macros in this file):

// src/modules/lingui/locale-fn.ts
// No Lingu

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