SvelteKit i18n: kansainvälistämisen käyttöönotto-opas

// src/lib/i18n.ts
import { register, init, getLocaleFromNavigator } from 'svelte-i18n';

// Register locale loaders (lazy-loaded)
register('en', () => import('../locales/en.json'));
register('de', () => import('../locales/de.json'));
register('ja', () => import('../locales/ja.json'));
register('es', () => import('../locales/es.json'));

init({
  fallbackLocale: 'en',
  initialLocale: getLocaleFromNavigator(),  // Auto-detect browser language
});

<!-- src/routes/+layout.svelte -->
<script>
  // Import i18n config — must run before any component renders
  import '../lib/i18n';
  import { isLoading } from 'svelte-i18n';
</script>

{#if $isLoading}
  <p>Loading translations...</p>
{:else}
  <slot />
{/if}

// src/routes/[lang]/+layout.ts
import { locale } from 'svelte-i18n';

export function load({ params }) {
  // Set the active locale from the URL parameter
  locale.set(params.lang);
  return {};
}

// src/routes/[lang]/+layout.svelte
<script>
  import '../../lib/i18n';
  import { isLoading } from 'svelte-i18n';
</script>

{#if $isLoading}
  <p>Loading...</p>
{:else}
  <slot />
{/if}

// src/locales/en.json
{
  "nav": {
    "home": "Home",
    "about": "About",
    "settings": "Settings"
  },
  "greeting": "Hello, {name}!",
  "cart": {
    "itemCount": "{count, plural, one {# item} other {# items}}"
  }
}

// src/locales/de.json
{
  "nav": {
    "home": "Startseite",
    "about": "Uber uns",
    "settings": "Einstellungen"
  },
  "greeting": "Hallo, {name}!",
  "cart": {
    "itemCount": "{count, plural, one {# Artikel} other {# Artikel}}"
  }
}

<script>
  import { _ } from 'svelte-i18n';
</script>

<h1>{$_('greeting', { values: { name: 'World' } })}</h1>

<nav>
  <a href="/">{$_('nav.home')}</a>
  <a href="/about">{$_('nav.about')}</a>
</nav>

<script>
  import { _, date, number, time } from 'svelte-i18n';
</script>

<!-- Simple string -->
<p>{$_('greeting', { values: { name: userName } })}</p>

<!-- ICU plurals — handled automatically -->
<p>{$_('cart.itemCount', { values: { count: 3 } })}</p>

<!-- Date formatting -->
<p>{$date(new Date(), { format: 'long' })}</p>

<!-- Number formatting -->
<p>{$number(1999.99, { style: 'currency', currency: 'USD' })}</p>

<script>
  import { locale, locales } from 'svelte-i18n';

  const LANGUAGE_NAMES = {
    en: 'English',
    de: 'Deutsch',
    ja: '日本語',
    es: 'Espanol',
  };
</script>

<select bind:value={$locale}>
  {#each $locales as loc}
    <option value={loc}>{LANGUAGE_NAMES[loc] ?? loc}</option>
  {/each}
</select>

// svelte-i18n uses ICU MessageFormat for plurals
// English:
{
  "items": "{count, plural, one {# item} other {# items}}"
}

// Arabic (6 forms):
{
  "items": "{count, plural, zero {no items} one {item} two {two items} few {# items} many {# items} other {# items}}"
}

// Japanese (1 form):
{
  "items": "{count, plural, other {#個のアイテム}}"
}

npm install svelte-i18n-locale-chain svelte-i18n

// src/lib/i18n.ts
import { initLocaleChain, setLocale } from 'svelte-i18n-locale-chain';

// Replace svelte-i18n's init + register with initLocaleChain
await initLocaleChain({
  loadMessages: (locale) =>
    import(`../locales/${locale}.json`).then(m => m.default),
  defaultLocale: 'en',
  initialLocale: 'pt-BR',
});

// Later, to change locale:
await setLocale('fr-CA');
// fr-CA user sees: fr-CA messages -> fr messages -> en messages
// No missing keys — deep-merged automatically

# In your IDE, ask your AI assistant:
> Translate src/locales/en.json to German, Japanese, and Spanish

✓ de.json created (1.2s)
✓ ja.json created (1.5s)
✓ es.json created (1.1s)

# Or use the CLI in CI/CD:
npx i18n-agent translate src/locales/en.json --lang de,ja,es

my-sveltekit-app/
├── src/
│   ├── lib/
│   │   └── i18n.ts              # i18n configuration
│   ├── locales/
│   │   ├── en.json              # Source language
│   │   ├── de.json              # German
│   │   ├── ja.json              # Japanese
│   │   └── es.json              # Spanish
│   └── routes/
│       ├── +layout.svelte       # Import i18n, guard isLoading
│       ├── +page.svelte
│       └── [lang]/              # Optional: locale-based routing
│           ├── +layout.ts       # Set locale from URL param
│           ├── +layout.svelte
│           └── +page.svelte
├── svelte.config.js
└── package.json