Translate your Vite and Lit website using Intlayer | Internationalisation (i18n)

Table of Contents

What is Intlayer?

Intlayer is an innovative, open-source internationalisation (i18n) library designed to simplify multilingual support in modern web applications.

With Intlayer, you can:

• Easily manage translations using declarative dictionaries at the component level.
• Dynamically localise metadata, routes, and content.
• Ensure TypeScript support with autogenerated types, improving autocompletion and error detection.
• Benefit from advanced features, like dynamic locale detection and switching.

See Application Template on GitHub.

Step-by-Step Guide to Set Up Intlayer in a Vite and Lit Application

Step 1: Install Dependencies

Install the necessary packages using npm:

npm install intlayer lit-intlayernpm install vite-intlayer --save-devnpx intlayer init

• intlayer

  The core package that provides internationalisation tools for configuration management, translation, content declaration, transpilation, and CLI commands.

• lit-intlayer The package that integrates Intlayer with Lit applications. It provides ReactiveController-based hooks (useIntlayer, useLocale, etc.) so that LitElements automatically re-render when the locale changes.

• vite-intlayer Includes the Vite plugin for integrating Intlayer with the Vite bundler, as well as middleware for detecting the user's preferred locale, managing cookies, and handling URL redirection.

Step 2: Configuration of your project

Create a config file to configure the languages of your application:

import { Locales, type IntlayerConfig } from "intlayer";

const config: IntlayerConfig = {
  internationalization: {
    locales: [
      Locales.ENGLISH,
      Locales.FRENCH,
      Locales.SPANISH,
      // Your other locales
    ],
    defaultLocale: Locales.ENGLISH,
  },
};

export default config;

Through this configuration file, you can set up localised URLs, middleware redirection, cookie names, the location and extension of your content declarations, disable Intlayer logs in the console, and more. For a complete list of available parameters, refer to the configuration documentation.

Step 3: Integrate Intlayer in Your Vite Configuration

Add the intlayer plugin into your configuration.

import { defineConfig } from "vite";
import { intlayer } from "vite-intlayer";

// https://vitejs.dev/config/
export default defineConfig({
  plugins: [intlayer()],
});

The intlayer() Vite plugin is used to integrate Intlayer with Vite. It ensures the building of content declaration files and monitors them in development mode. It defines Intlayer environment variables within the Vite application. Additionally, it provides aliases to optimise performance.

Step 4: Bootstrap Intlayer in your entry point

Call installIntlayer() before any custom elements are registered so that the global locale singleton is ready when the first element connects.

import { installIntlayer } from "lit-intlayer";// Must be called before any LitElement is connected to the DOM.installIntlayer();// Import and register your custom elements.import "./my-element.js";

If you also use md() content declarations (Markdown), install the markdown renderer as well:

import { installIntlayer, installIntlayerMarkdown } from "lit-intlayer";installIntlayer();installIntlayerMarkdown();import "./my-element.js";

Step 5: Declare Your Content

Create and manage your content declarations to store translations:

import { t, type Dictionary } from "intlayer";

const appContent = {
  key: "app",
  content: {
    title: "Vite + Lit",

    viteLogo: t({
      en: "Vite logo",
      fr: "Logo Vite",
      es: "Logo Vite",
    }),
    litLogo: t({
      en: "Lit logo",
      fr: "Logo Lit",
      es: "Logo Lit",
    }),

    count: t({
      en: "count is {{count}}",
      fr: "le compte est {{count}}",
      es: "el recuento es {{count}}",
    }),

    readTheDocs: t({
      en: "Click on the Vite and Lit logos to learn more",
      fr: "Cliquez sur les logos Vite et Lit pour en savoir plus",
      es: "Haga clic en los logotipos de Vite y Lit para obtener más información",
    }),
  },
} satisfies Dictionary;

export default appContent;

Your content declarations can be defined anywhere in your application as soon as they are included in the contentDir directory (by default, ./src). And match the content declaration file extension (by default, .content.{json,ts,tsx,js,jsx,mjs,cjs}).

For more details, refer to the content declaration documentation.

Step 6: Utilise Intlayer in Your LitElement

Use useIntlayer inside a LitElement. It returns a ReactiveController proxy that automatically triggers re-renders whenever the active locale changes - no extra setup required.

import { LitElement, html } from "lit";import { customElement, property } from "lit/decorators.js";import { useIntlayer } from "lit-intlayer";@customElement("my-element")export class MyElement extends LitElement {  @property({ type: Number })  count = 0;  // useIntlayer registers itself as a ReactiveController.  // The element re-renders automatically when the locale changes.  private content = useIntlayer(this, "app");  override render() {    const { content } = this;    return html`      <h1>${content.title}</h1>      <img src="/vite.svg" alt=${content.viteLogo.value} />      <img src="/lit.svg" alt=${content.litLogo.value} />      <button @click=${() => this.count++}>        ${content.count({ count: this.count })}      </button>      <p>${content.readTheDocs}</p>    `;  }}

When you need the translated string in a native HTML attribute (e.g. alt, aria-label, title), you can use the translated string in several ways:

typescript

Copy content

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html`<img alt=${content.viteLogo.value} />`;html`<img alt=${content.viteLogo.toString()} />`;html`<img alt=${String(content.viteLogo)} />`;

(Optional) Step 7: Change the language of your content

To change the language of your content, use the setLocale method exposed by the useLocale controller.

import { LitElement, html } from "lit";import { customElement } from "lit/decorators.js";import { getLocaleName } from "intlayer";import { useLocale } from "lit-intlayer";@customElement("locale-switcher")export class LocaleSwitcher extends LitElement {  private locale = useLocale(this);  private _onChange(e: Event) {    const select = e.target as HTMLSelectElement;    this.locale.setLocale(select.value as any);  }  override render() {    return html`      <select @change=${this._onChange}>        ${this.locale.availableLocales.map(          (loc) => html`            <option value=${loc} ?selected=${loc === this.locale.locale}>              ${getLocaleName(loc)}            </option>          `        )}      </select>    `;  }}

(Optional) Step 8: Render Markdown and HTML content

Intlayer supports md() and html() content declarations. In Lit, compiled output is injected as raw HTML via the unsafeHTML directive.

Render the compiled HTML in your element:

import { LitElement, html } from "lit";import { customElement } from "lit/decorators.js";import { unsafeHTML } from "lit/directives/unsafe-html.js";import { useIntlayer } from "lit-intlayer";import { compileMarkdown } from "lit-intlayer/markdown";@customElement("my-element")export class MyElement extends LitElement {  private content = useIntlayer(this, "app");  override render() {    return html`      <div class="edit-note">        ${unsafeHTML(compileMarkdown(String(this.content.editNote)))}      </div>    `;  }}

TIP

String(content.editNote) calls toString() on the IntlayerNode, which returns the raw Markdown string. Pass it to compileMarkdown to get an HTML string, then render it with Lit's unsafeHTML directive.

(Optional) Step 9: Add localised Routing to your application

To make unique routes for each language (useful for SEO), you can use a client-side router alongside Intlayer's localeMap / localeFlatMap helpers, and the intlayerProxy Vite plugin for server-side locale detection.

First, add intlayerProxy to your Vite config:

Note that to use intlayerProxy in production, you need to move vite-intlayer from devDependencies to dependencies.

import { defineConfig } from "vite";
import { intlayer, intlayerProxy } from "vite-intlayer";

export default defineConfig({
  plugins: [
    intlayerProxy(), // should be placed first
    intlayer(),
  ],
});

(Optional) Step 10: Change the URL when the locale changes

To update the browser URL when the locale changes, use useRewriteURL alongside the locale switcher:

import { LitElement, html } from "lit";import { customElement } from "lit/decorators.js";import { getLocaleName, getLocalizedUrl } from "intlayer";import { useLocale, useRewriteURL } from "lit-intlayer";@customElement("locale-switcher")export class LocaleSwitcher extends LitElement {  private locale = useLocale(this);  // Automatically rewrites the current URL when the locale changes.  private _rewriteURL = useRewriteURL(this);  private _onChange(e: Event) {    const select = e.target as HTMLSelectElement;    this.locale.setLocale(select.value as any);  }  override render() {    return html`      <select @change=${this._onChange}>        ${this.locale.availableLocales.map(          (loc) => html`            <option value=${loc} ?selected=${loc === this.locale.locale}>              ${getLocaleName(loc)}            </option>          `        )}      </select>    `;  }}

(Optional) Step 11: Switch the HTML Language and Direction Attributes

Update the <html> tag's lang and dir attributes to match the current locale for accessibility and SEO.

import { LitElement, html } from "lit";import { customElement } from "lit/decorators.js";import { getHTMLTextDir } from "intlayer";import { useLocale } from "lit-intlayer";@customElement("my-element")export class MyElement extends LitElement {  private locale = useLocale(this, {    onLocaleChange: (loc) => {      document.documentElement.lang = loc;      document.documentElement.dir = getHTMLTextDir(loc);    },  });  override render() {    return html`<!-- your content -->`;  }}

(Optional) Step 12: Extract the content of your components

If you have an existing codebase, transforming thousands of files can be time-consuming.

To ease this process, Intlayer proposes a compiler / extractor to transform your components and extract the content.

To set it up, you can add a compiler section in your intlayer.config.ts file:

import { type IntlayerConfig } from "intlayer";const config: IntlayerConfig = {  // ... Rest of your config  compiler: {    /**     * Indicates if the compiler should be enabled.     */    enabled: true,    /**     * Defines the output files path     */    output: ({ fileName, extension }) => `./${fileName}${extension}`,    /**     * Indicates if the components should be saved after being transformed.     *     * - If `true`, the compiler will rewrite the component file in the disk. So the transformation will be permanent, and the compiler will skip the transformation for the next process. That way, the compiler can transform the app, and then it can be removed.     *     * - If `false`, the compiler will inject the `useIntlayer()` function call into the code in the build output only, and keep the base codebase intact. The transformation will be done only in memory.     */    saveComponents: false,    /**     * Dictionary key prefix     */    dictionaryKeyPrefix: "",  },};export default config;

(Optional) Sitemap and robots.txt (build-time)

Intlayer includes formatters such as generateSitemap and getMultilingualUrls that produce crawler-ready multilingual sitemap.xml and robots.txt output you can write into your project’s public/ folder. In practice you run a small Node script before Vite (for example predev / prebuild npm hooks) so those files exist when you build or serve the app.

Sitemap

Intlayer’s sitemap generator respects your locale setup and includes the usual metadata for crawlers.

The generated sitemap supports the xhtml:link namespace (hreflang XML extensions). Unlike basic generators that only emit flat URLs, Intlayer wires bidirectional links between every localized variant of each page (for example /about, /fr/about, or /about?lang=fr, depending on your routing mode), which helps search engines relate localized URLs.

Robots.txt

Use getMultilingualUrls so Disallow entries cover every localized spelling of sensitive paths.

1. Add generate-seo.mjs at the project root

import fs from "fs";import path from "path";import { fileURLToPath } from "url";import { generateSitemap, getMultilingualUrls } from "intlayer";const __dirname = path.dirname(fileURLToPath(import.meta.url));const SITE_URL = (process.env.SITE_URL || "http://localhost:5173").replace(  /\/$/,  "");const pathList = [  { path: "/", changefreq: "daily", priority: 1.0 },  { path: "/about", changefreq: "monthly", priority: 0.7 },];const sitemapXml = generateSitemap(pathList, { siteUrl: SITE_URL });fs.writeFileSync(path.join(__dirname, "public", "sitemap.xml"), sitemapXml);const getAllMultilingualUrls = (urls) =>  urls.flatMap((url) => Object.values(getMultilingualUrls(url)));const disallowedPaths = getAllMultilingualUrls(["/admin", "/private"]);const robotsTxt = [  "User-agent: *",  "Allow: /",  ...disallowedPaths.map((path) => `Disallow: ${path}`),  "",  `Sitemap: ${SITE_URL}/sitemap.xml`,].join("\n");fs.writeFileSync(path.join(__dirname, "public", "robots.txt"), robotsTxt);console.log("SEO files generated successfully.");

intlayer must be installed so the script can import it. Set SITE_URL in the environment for production (for example in CI).

Prefer generate-seo.mjs for Node ESM. If you use generate-seo.js instead, ensure "type": "module" is set in package.json, or run Node with ESM enabled.

2. Run the script before Vite

{  "scripts": {    "dev": "vite",    "prebuild": "node generate-seo.mjs",    "build": "vite build",    "preview": "vite preview"  }}

Adjust if you use pnpm or yarn. You can also invoke the same script from CI or another step if that fits your workflow.

Configure TypeScript

Ensure your TypeScript configuration includes the autogenerated types.

{  "compilerOptions": {    // ...    "experimentalDecorators": true,    "useDefineForClassFields": false,  },  "include": ["src", ".intlayer/**/*.ts"],}

experimentalDecorators and useDefineForClassFields: false are required by Lit for decorator support.

Git Configuration

It is recommended to ignore the files generated by Intlayer. This allows you to avoid committing them to your Git repository.

To do this, you can add the following instructions to your .gitignore file:

# Ignore the files generated by Intlayer.intlayer

VS Code Extension

To improve your development experience with Intlayer, you can install the official Intlayer VS Code Extension.

Install from the VS Code Marketplace

This extension provides:

• Autocompletion for translation keys.
• Real-time error detection for missing translations.
• Inline previews of translated content.
• Quick actions to easily create and update translations.

For more details on how to use the extension, refer to the Intlayer VS Code Extension documentation.

Go Further

To go further, you can implement the visual editor or externalise your content using the CMS.