Internationalization (i18n)

The project supports multiple languages out of the box. The frontend uses Paraglide.js  for translations, while the backend uses Spring’s MessageSource for error code resolution.

Supported Languages

• English (en) - source language
• Serbian (sr)

Frontend

Message Files

Translations live in JSON files under messages/:

messages/
  en.json
  sr.json

Each file contains key-value pairs:

{
  "auth_signIn": "Sign In",
  "auth_signUp": "Sign Up",
  "API_ERROR_USER_NOT_FOUND": "User not found"
}

Keys use underscore-separated naming (e.g., auth_signIn, admin_userCreate, profile_changePassword).

Using Translations in Components

Import the generated message functions and call them:

<script lang="ts">
  import * as m from '$lib/paraglide/messages.js';
</script>
 
<h1>{m.auth_signIn()}</h1>

Messages are functions, not strings. This lets Paraglide swap the language at runtime.

Language Routing

Every URL is prefixed with the language tag (e.g., /en/profile, /sr/profile). This is configured in src/lib/i18n.ts:

export const i18n = createI18n(runtime, {
  prefixDefaultLanguage: 'always',
});

The i18n.handle() middleware in hooks.server.ts detects the language from the URL and sets it for the request. The i18n.reroute() in hooks.ts handles client-side navigation.

Language Switcher

The language switcher component in src/lib/components/navbar/language-switcher.svelte uses i18n.route() to get the canonical path and i18n.resolveRoute() to navigate to the same page in a different language.

Backend

The backend does not serve translated user-facing content. Instead, it uses MessageSource to resolve error keys from error-codes.properties into error constants (e.g., user.notFound becomes API_ERROR_USER_NOT_FOUND). The frontend then maps these constants to translated messages.

See the Error Handling section for more details.

Adding a New Language

1. Create a new message file messages/{languageCode}.json with all the keys from en.json translated.

2. Add the language tag to project.inlang/settings.json:

   "languageTags": ["en", "sr", "fr"]

3. Add a label for the language in all message files:

   "language_french": "French"

4. The Paraglide Vite plugin will regenerate the runtime files automatically on the next build or dev server start. The language will appear in the language switcher and be available for routing.