Paraglide JS

Compiler-based i18n library. Compiles translation files (JSON/inlang) into tree-shakable JS functions. Each message becomes a function — only used messages are bundled.

Architecture

messages/          ← Translation source files (JSON)
  en.json
  de.json
project.inlang/
  settings.json    ← Inlang project config (locales, plugins, sourceLanguageTag)
src/paraglide/     ← Compiler output (generated, gitignored)
  messages.js      ← Compiled message functions
  runtime.js       ← getLocale(), setLocale(), locales, sourceLocale
  server.js        ← paraglideMiddleware() for SSR

Compiler runs via bundler plugin (Vite/Webpack/Rollup) or CLI. Output is always ./src/paraglide/.

Core Usage

Message Functions

import * as m from "./paraglide/messages.js";

// Simple message — no params
m.hello_world();  // "Hello World"

// Parameterized message
m.greeting({ name: "Alice" });  // "Hello, Alice!"

Messages are defined in JSON translation files:

// messages/en.json
{
  "hello_world": "Hello World",
  "greeting": "Hello, {name}!"
}

Runtime

import { getLocale, setLocale, locales, sourceLocale } from "./paraglide/runtime.js";

getLocale();        // Current locale, e.g. "en"
setLocale("de");    // Change locale (triggers re-render in frameworks)
locales;            // ["en", "de", "fr"] — all configured locales
sourceLocale;       // "en" — the source language

Message functions automatically use getLocale(). Override per-call:

m.greeting({ name: "Alice" }, { locale: "de" });  // Force German

Overriding `getLocale` (Critical for SSR)

getLocale and setLocale must be overridden in the inlang project settings for any real app. The default is a simple global variable — not suitable for SSR (concurrent requests share state).

// project.inlang/settings.json
{
  "baseLocale": "en",
  "locales": ["en", "de", "fr"],
  "plugin": "https://cdn.jsdelivr.net/npm/@inlang/paraglide-js@2/dist/plugin.js",
  "$imports": {
    "default": "https://cdn.jsdelivr.net/npm/@anthropic-ai/sdk@latest/dist/index.js"
  }
}

Override via compilerOptions.emitGitIgnore, compilerOptions.outputStructure, etc. in settings.

For SSR, use paraglideMiddleware() which handles request-scoped locale via AsyncLocalStorage. See references/rendering.md.

Strategy (Locale Detection)

Strategy is an ordered fallback chain that determines the locale. Configure via compiler options.

import { defineCompilerOptions } from "@inlang/paraglide-js";

export default defineCompilerOptions({
  strategy: ["url", "cookie", "preferredLanguage", "baseLocale"],
  urlPatterns: [{ pattern: "/:locale(en|de|fr)/:path*" }],
  cookieName: "NEXT_LOCALE",
});

Common strategies: url, cookie, preferredLanguage, globalVariable, localStorage, baseLocale.

For full strategy configuration, URL patterns, custom strategies, and middleware setup, see references/routing-and-strategy.md.

Key Integration Patterns

Bundler Plugin (Recommended)

// vite.config.ts
import { paraglideVitePlugin } from "@inlang/paraglide-js";

export default defineConfig({
  plugins: [paraglideVitePlugin()],
});

Also available: paraglideWebpackPlugin(), paraglideRollupPlugin(), paraglideRolldownPlugin().

For full setup, CLI compilation, and TypeScript config, see references/setup.md.

SSR Middleware

import { paraglideMiddleware } from "./paraglide/server.js";

app.use((req, res, next) => {
  paraglideMiddleware(req, ({ request, locale }) => {
    // locale is detected, getLocale() works in this scope
    return next();
  });
});

For SSR, SSG, page discovery, and hreflang, see references/rendering.md.

Reference Files

Read these as needed based on the task:

references/setup.md — Installation, bundler plugins, CLI/programmatic compilation, TypeScript config, translation file formats, inlang project settings
references/routing-and-strategy.md — Strategy configuration, URL patterns, custom strategies, middleware for all frameworks (Next.js, SvelteKit, Astro, Express, Hono, etc.), client-side redirects
references/rendering.md — SSR with paraglideMiddleware, SSG page discovery, setting locale for static pages, hreflang SEO tags
references/advanced.md — Variants (pluralization/gendering), number/date formatting, rich text markup, message keys, objects/arrays, multi-tenancy, monorepo setup, common errors

paraglide-js