paragone

From Greek "παράγλωσσος" (paraglossus) - multilingual, speaking many languages

Modern internationalization (i18n) library for SvelteKit with Svelte 5 Runes and remotes support.

Features

• Cookie-based with automatic browser language detection
• Route-specific translation files
• Reactive with Svelte 5 Runes ($derived)
• Type-safe TypeScript support
• Minimal setup required
• Remote functions for language switching with command()
• Nested keys support (nav.home, button.save)
• Variable interpolation ({{name}}, {{count}})
• Global + Route translations

Installation

npm install paragone

pnpm add paragone

bun add paragone

Quick Start

1. Setup hooks (detect language)

Create or update src/hooks.server.ts:

import type { Handle } from '@sveltejs/kit';
import { getLanguage } from 'paragone';

export const handle: Handle = async ({ event, resolve }) => {
  const language = getLanguage(
    event.cookies,
    event.request.headers.get('accept-language')
  );
  
  event.locals.language = language;
  
  return resolve(event);
};

2. Define your types

Update src/app.d.ts:

declare global {
  namespace App {
    interface Locals {
      language: string;
    }
  }
}

export {};

3. Create translation file

Create src/routes/+page/locale.json:

{
  "en": {
    "title": "Welcome",
    "greeting": "Hello, {{name}}!",
    "button": {
      "login": "Login",
      "signup": "Sign up"
    }
  },
  "de": {
    "title": "Willkommen",
    "greeting": "Hallo, {{name}}!",
    "button": {
      "login": "Anmelden",
      "signup": "Registrieren"
    }
  }
}

4. Use in server load function (yes, you can use your locals also server-side)

src/routes/+page.server.ts:

import type { PageServerLoad } from './$types';
import { I18n } from 'paragone';
import * as locale from './locale.json';

export const load: PageServerLoad = async ({ locals }) => {
  const { t } = new I18n(locale, locals.language);
  
  return {
    title: t('title'),
    language: locals.language
  };
};

5. Use in Svelte component

src/routes/+page.svelte:

<script lang="ts">
  import { I18n } from 'paragone';
  import * as locale from './locale.json';
  
  let { data } = $props();
  const { t } = $derived(new I18n(locale, data.language));
</script>

<h1>{t('title')}</h1>
<p>{t('greeting', { name: 'Sarah' })}</p>
<button>{t('button.login')}</button>
<button>{t('button.signup')}</button>

6. Switch language

First, create src/lib/changeLanguage.ts:

import { command, getRequestEvent } from "$app/server";
import { setLanguage } from "paragone";
import z from "zod";

/**
 * Remote function to change the user's language preference
 * @example await changeLanguage('de')
 */
export const changeLanguage = command(
  z.string().min(2).max(10),
  async (language) => {
    const event = getRequestEvent();
    setLanguage(event.cookies, language);
    return { success: true, language };
  }
);

Then use it in src/routes/+page.svelte:

<script lang="ts">
  import { invalidateAll } from '$app/navigation';
  import { changeLanguage } from '$lib/changeLanguage';
  
  async function switchLanguage(lang: string) {
    await changeLanguage(lang);
    await invalidateAll();
  }
</script>

<button onclick={() => switchLanguage('en')}>English</button>
<button onclick={() => switchLanguage('de')}>Deutsch</button>

Note: changeLanguage is not exported from paragone because it uses $app/server which only works in the context of your SvelteKit project. See docs/REMOTE_FUNCTIONS.md for alternative implementations (Form Actions, API Routes, etc.).

API Reference

I18n Class

const i18n = new I18n(translations, locale);

Parameters

• translations: Object with language keys containing translation objects
• locale: Current language code (e.g., 'en', 'de')

Methods

t(key: string, vars?: Record<string, string | number>): string

Get translation by key with optional variable replacement.

t('title')                           // "Welcome"
t('greeting', { name: 'Mike' })      // "Hello, Mike!"
t('button.login')                    // "Login" (nested keys)

has(key: string): boolean

Check if translation key exists.

if (i18n.has('optional.key')) {
  console.log(t('optional.key'));
}

getLocale(): string

Get current locale.

const locale = i18n.getLocale(); // "en" or "de"

Helper Functions

getLanguage(cookies: Cookies, acceptLanguage: string | null): string

Get language from cookie or detect from browser's Accept-Language header.

import { getLanguage } from 'paragone';

const language = getLanguage(
  event.cookies,
  event.request.headers.get('accept-language')
);

setLanguage(cookies: Cookies, language: string): void

Set language to cookie.

import { setLanguage } from 'paragone';

setLanguage(cookies, 'de');

Remote Function

changeLanguage(language: string)

Change user's language preference. You need to implement this in your own project because it uses SvelteKit's command() which cannot be exported from a library.

Create src/lib/changeLanguage.ts:

import { command, getRequestEvent } from "$app/server";
import { setLanguage } from "paragone";
import z from "zod";

export const changeLanguage = command(
  z.string().min(2).max(10),
  async (language) => {
    const event = getRequestEvent();
    setLanguage(event.cookies, language);
    return { success: true, language };
  }
);

Usage:

import { changeLanguage } from '$lib/changeLanguage';

await changeLanguage('de');
await invalidateAll(); // Reload page data

See docs/REMOTE_FUNCTIONS.md for alternative implementations (Form Actions, API Routes).

Usage Patterns

Global Translations

For app-wide translations (navigation, common buttons, etc.):

src/lib/i18n/locale.json:

{
  "en": {
    "nav": {
      "home": "Home",
      "about": "About",
      "contact": "Contact"
    }
  },
  "de": {
    "nav": {
      "home": "Startseite",
      "about": "Über uns",
      "contact": "Kontakt"
    }
  }
}

Use in layout:

import * as globalLocale from '$lib/i18n/locale.json';

const { t } = new I18n(globalLocale, locals.language);

Merge Global + Route Translations

import * as globalLocale from '$lib/i18n/locale.json';
import * as pageLocale from './locale.json';

const combined = {
  en: { ...globalLocale.en, ...pageLocale.en },
  de: { ...globalLocale.de, ...pageLocale.de }
};

const { t } = new I18n(combined, locals.language);

Use in Your Own Remote Functions (Commands)

import { command } from '$app/server';
import { I18n } from 'paragone';
import * as locale from './locale.json';
import z from 'zod';

export const saveData = command(z.object({ name: z.string() }), async (data, { locals }) => {
  const { t } = new I18n(locale, locals.language);
  
  // ... your logic
  
  return {
    success: true,
    message: t('messages.saved')
  };
});

Note: Remote functions must be defined in your project, not imported from paragone.

Configuration

Edit the constants in your local copy if needed:

const COOKIE_NAME = 'language';
const DEFAULT_LANGUAGE = 'en';
const SUPPORTED_LANGUAGES = ['en', 'de', 'fr', 'es'];

Cookie settings:

{
  path: '/',
  maxAge: 60 * 60 * 24 * 365, // 1 year
  sameSite: 'lax',
  httpOnly: false
}

Browser Language Detection

The system automatically detects the user's browser language on first visit:

1. Checks if language cookie exists
2. If not, reads Accept-Language header
3. Matches against supported languages
4. Falls back to default (en)

Example header: Accept-Language: de-DE,de;q=0.9,en;q=0.8 → Returns de (if supported)

Examples

Check out the examples folder for complete working examples:

• Basic Usage - Simple translation setup
• Multi-Language - Multiple languages with switcher
• Nested Keys - Complex translation structures
• Remote Functions - Using translations in commands

Testing

npm test

Best Practices

✅ DO

• Use $derived in components for reactivity
• Structure keys hierarchically
• Use variables for dynamic content
• Call invalidateAll() after language change

❌ DON'T

• Avoid creating new i18n instances in loops
• Don't hardcode translatable strings
• Pass language from server to client

How It Works

1. Request comes in
   ↓
2. hooks.server.ts calls getLanguage()
   ↓
3. Checks: Cookie → Browser → Default ('en')
   ↓
4. Stores in locals.language
   ↓
5. Page loads with language
   ↓
6. User clicks language button
   ↓
7. changeLanguage() sets cookie
   ↓
8. invalidateAll() reloads with new language

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

1. Fork the repository
2. Create your feature branch (git checkout -b feature/amazing-feature)
3. Commit your changes (git commit -m 'Add some amazing feature')
4. Push to the branch (git push origin feature/amazing-feature)
5. Open a Pull Request

License

MIT © Tayfun Guelcan

Links

• Documentation
• Examples
• Issues
• NPM Package

Made for the Svelte community

And by the way For those who think, “Oh no, not another language library!”, the answer is: Shut up and use it!