Impertio Studio Fluent I18N Claude Skill Package Fluent Agents Project Scaffolder

fluent-agents-project-scaffolder

Quick Reference

Generated File Manifest

Required npm Dependencies

Peer Dependencies (must already exist)

Critical Warnings

ALWAYS install all three core packages together -- they form an inseparable stack for React-based Fluent projects.

ALWAYS generate FTL files with a trailing newline -- the FTL parser expects it.

ALWAYS check addResource() return value for parse errors -- silent failures cause missing translations at runtime.

NEVER generate FTL files with concatenated strings or template literals at runtime -- ALWAYS serve .ftl files statically via fetch().

NEVER create ReactLocalization inside a render function without memoization -- this causes full re-render of all localized components on every state change.

NEVER skip the async loading pattern -- bundling FTL strings into JavaScript defeats the purpose of locale-based code splitting.

NEVER hardcode navigator.languages as the only locale source -- ALWAYS pass it through negotiateLanguages() to handle fallbacks correctly.

Decision Tree: Which Scaffold?

What type of project?
├── React app (Vite, CRA, Next.js client)
│   ├── New project → Full scaffold (all files)
│   └── Existing project → Integration scaffold (l10n/ + FTL files only)
│
├── Vanilla JS (no framework)
│   └── Skip: LocalizedApp.tsx, @fluent/react
│       Generate: l10n/index.ts (vanilla), FTL files, bundle setup
│       Install: @fluent/bundle + @fluent/langneg only
│
└── Node.js server (Express, Fastify)
    └── Skip: LocalizedApp.tsx, @fluent/react, localStorage
        Generate: l10n/index.ts (server), FTL files, bundle setup
        Install: @fluent/bundle + @fluent/langneg only
        Use: acceptedLanguages() from @fluent/langneg for HTTP headers

Scaffolding Steps

Step 1: Install Dependencies

# React project (ALWAYS all three)
npm install @fluent/bundle @fluent/react @fluent/langneg

# Vanilla JS or Node.js (no React bindings needed)
npm install @fluent/bundle @fluent/langneg

Step 2: Create Directory Structure

project-root/
├── public/
│   └── locales/
│       ├── en-US/
│       │   └── main.ftl
│       ├── fr/
│       │   └── main.ftl
│       └── de/
│           └── main.ftl
└── src/
    ├── l10n/
    │   ├── index.ts
    │   ├── bundles.ts
    │   └── switch.ts
    └── components/
        └── LocalizedApp.tsx

ALWAYS use BCP 47 locale identifiers for directory names: en-US, fr, de-DE.

ALWAYS place FTL files under public/locales/ for static serving via fetch().

Step 3: Generate FTL Files

ALWAYS seed each locale with these baseline messages:

# public/locales/en-US/main.ftl
app-title = My Application
nav-home = Home
nav-settings = Settings
welcome-user = Welcome, { $userName }!
items-count = { $count ->
    [one] You have { $count } item.
   *[other] You have { $count } items.
}
loading = Loading...
error-generic = Something went wrong. Please try again.

ALWAYS use kebab-case for message IDs.

ALWAYS include a plural selector example to establish the pattern for translators.

ALWAYS include loading and error-generic messages -- every app needs them.

Step 4: Generate l10n/index.ts

This is the entry point for localization. It exports constants and the initialization function:

// src/l10n/index.ts
import { negotiateLanguages } from "@fluent/langneg";
import { ReactLocalization } from "@fluent/react";
import { createBundles } from "./bundles";

export const AVAILABLE_LOCALES = ["en-US", "fr", "de"];
export const DEFAULT_LOCALE = "en-US";

export async function initLocalization(): Promise<ReactLocalization> {
  const negotiated = negotiateLanguages(
    navigator.languages,
    AVAILABLE_LOCALES,
    { defaultLocale: DEFAULT_LOCALE }
  );

  const bundles = await createBundles(negotiated);
  return new ReactLocalization(bundles);
}

Step 5: Generate l10n/bundles.ts

Handles FTL fetching, FluentBundle creation, and error logging:

// src/l10n/bundles.ts
import { FluentBundle, FluentResource } from "@fluent/bundle";

async function fetchMessages(locale: string): Promise<string> {
  const response = await fetch(`/locales/${locale}/main.ftl`);
  if (!response.ok) {
    throw new Error(`Failed to fetch FTL for ${locale}: ${response.status}`);
  }
  return response.text();
}

function createBundle(locale: string, messages: string): FluentBundle {
  const bundle = new FluentBundle(locale);
  const resource = new FluentResource(messages);
  const errors = bundle.addResource(resource);
  if (errors.length) {
    errors.forEach((e) => console.warn(`FTL parse error in ${locale}:`, e));
  }
  return bundle;
}

export async function createBundles(
  locales: string[]
): Promise<FluentBundle[]> {
  return Promise.all(
    locales.map(async (locale) => {
      const messages = await fetchMessages(locale);
      return createBundle(locale, messages);
    })
  );
}

Step 6: Generate l10n/switch.ts

Provides locale switching with localStorage persistence:

// src/l10n/switch.ts
import { negotiateLanguages } from "@fluent/langneg";
import { ReactLocalization } from "@fluent/react";
import { AVAILABLE_LOCALES, DEFAULT_LOCALE } from "./index";
import { createBundles } from "./bundles";

const LOCALE_STORAGE_KEY = "preferred-locale";

export function getSavedLocale(): string | null {
  return localStorage.getItem(LOCALE_STORAGE_KEY);
}

export function saveLocalePreference(locale: string): void {
  localStorage.setItem(LOCALE_STORAGE_KEY, locale);
}

export async function switchLocale(
  locale: string
): Promise<ReactLocalization> {
  saveLocalePreference(locale);
  const negotiated = negotiateLanguages(
    [locale],
    AVAILABLE_LOCALES,
    { defaultLocale: DEFAULT_LOCALE }
  );
  const bundles = await createBundles(negotiated);
  return new ReactLocalization(bundles);
}

Step 7: Generate LocalizedApp.tsx

The root wrapper component with async init and loading state:

// src/components/LocalizedApp.tsx
import React, { useState, useEffect, useCallback } from "react";
import { LocalizationProvider } from "@fluent/react";
import { ReactLocalization } from "@fluent/react";
import { initLocalization } from "../l10n";
import { switchLocale, getSavedLocale } from "../l10n/switch";
import { negotiateLanguages } from "@fluent/langneg";
import { AVAILABLE_LOCALES, DEFAULT_LOCALE } from "../l10n";

interface Props {
  children: React.ReactNode;
}

export function LocalizedApp({ children }: Props) {
  const [l10n, setL10n] = useState<ReactLocalization | null>(null);

  useEffect(() => {
    const saved = getSavedLocale();
    if (saved) {
      switchLocale(saved).then(setL10n);
    } else {
      initLocalization().then(setL10n);
    }
  }, []);

  const handleLocaleSwitch = useCallback(async (locale: string) => {
    const newL10n = await switchLocale(locale);
    setL10n(newL10n);
  }, []);

  if (!l10n) {
    return <div>Loading...</div>;
  }

  return (
    <LocalizationProvider l10n={l10n}>
      {children}
    </LocalizationProvider>
  );
}

Step 8: Wire Into App Entry Point

// src/App.tsx or src/main.tsx
import React from "react";
import { LocalizedApp } from "./components/LocalizedApp";

function App() {
  return (
    <LocalizedApp>
      {/* Your application components here */}
    </LocalizedApp>
  );
}

export default App;

Vanilla JS Scaffold

For projects without React, skip @fluent/react and use the bundle API directly:

// src/l10n/index.ts (vanilla JS variant)
import { FluentBundle, FluentResource } from "@fluent/bundle";
import { negotiateLanguages } from "@fluent/langneg";

const AVAILABLE_LOCALES = ["en-US", "fr", "de"];
const DEFAULT_LOCALE = "en-US";

let currentBundle: FluentBundle | null = null;

export async function initLocalization(): Promise<FluentBundle> {
  const negotiated = negotiateLanguages(
    navigator.languages,
    AVAILABLE_LOCALES,
    { defaultLocale: DEFAULT_LOCALE }
  );

  const locale = negotiated[0];
  const response = await fetch(`/locales/${locale}/main.ftl`);
  const messages = await response.text();
  const bundle = new FluentBundle(locale);
  const errors = bundle.addResource(new FluentResource(messages));
  if (errors.length) {
    errors.forEach((e) => console.warn(`FTL error:`, e));
  }
  currentBundle = bundle;
  return bundle;
}

export function t(id: string, args?: Record<string, string | number>): string {
  if (!currentBundle) return id;
  const msg = currentBundle.getMessage(id);
  if (!msg?.value) return id;
  const errors: Error[] = [];
  return currentBundle.formatPattern(msg.value, args, errors);
}

Node.js Server Scaffold

For Express/Fastify, use acceptedLanguages() instead of navigator.languages:

// src/l10n/server.ts
import { FluentBundle, FluentResource } from "@fluent/bundle";
import { negotiateLanguages, acceptedLanguages } from "@fluent/langneg";
import { readFileSync } from "fs";
import { join } from "path";

const AVAILABLE_LOCALES = ["en-US", "fr", "de"];
const DEFAULT_LOCALE = "en-US";

const bundleCache = new Map<string, FluentBundle>();

function loadBundle(locale: string): FluentBundle {
  if (bundleCache.has(locale)) return bundleCache.get(locale)!;
  const ftlPath = join(__dirname, `../../locales/${locale}/main.ftl`);
  const messages = readFileSync(ftlPath, "utf-8");
  const bundle = new FluentBundle(locale);
  const errors = bundle.addResource(new FluentResource(messages));
  if (errors.length) {
    errors.forEach((e) => console.warn(`FTL error in ${locale}:`, e));
  }
  bundleCache.set(locale, bundle);
  return bundle;
}

export function getLocalization(acceptLanguageHeader: string): FluentBundle {
  const requested = acceptedLanguages(acceptLanguageHeader);
  const negotiated = negotiateLanguages(requested, AVAILABLE_LOCALES, {
    defaultLocale: DEFAULT_LOCALE,
  });
  return loadBundle(negotiated[0]);
}

Checklist: Verify Scaffold Completeness

After generating all files, verify:

• npm install succeeds with no peer dependency warnings
• All FTL files end with a trailing newline
• AVAILABLE_LOCALES array matches directory names under public/locales/
• DEFAULT_LOCALE is included in AVAILABLE_LOCALES
• Every locale directory has the same set of FTL files
• addResource() errors are logged, not silently ignored
• LocalizedApp renders a loading state while translations load
• Locale switching creates a NEW ReactLocalization instance (not mutating)

Reference Links

• references/methods.md -- File templates, dependency list, configuration reference
• references/examples.md -- Complete scaffolded project output with all files
• references/anti-patterns.md -- Scaffolding mistakes to avoid

Official Sources

• https://github.com/projectfluent/fluent.js/wiki/React-Tutorial
• https://github.com/projectfluent/fluent.js/tree/main/fluent-bundle
• https://github.com/projectfluent/fluent.js/tree/main/fluent-react
• https://github.com/projectfluent/fluent.js/tree/main/fluent-langneg

Source: Impertio-Studio/Fluent-i18n-Claude-Skill-Package — distributed by TomeVault.