How do you serve content in multiple languages?

HTML

The short answer

Use a combination of the HTTP Accept-Language header (to detect the user's preferred language), URL-based language routing (like /en/ or /es/), translation files with a library like i18next, and proper HTML attributes (lang, hreflang). Let users switch languages manually — never rely only on automatic detection.

Detecting the user's language

From the HTTP header:

// Server-side (Express)
app.get('/', (req, res) => {
  const language = req.headers['accept-language']; // "en-US,en;q=0.9,es;q=0.8"
  const preferred = language.split(',')[0].split('-')[0]; // "en"
  res.redirect(`/${preferred}/`);
});

From the browser:

const language = navigator.language; // "en-US"
const languages = navigator.languages; // ["en-US", "en", "es"]

Translation files

Store translations as key-value pairs:

// en.json
{ "greeting": "Hello", "cart": "Shopping Cart", "checkout": "Checkout" }
// es.json
{ "greeting": "Hola", "cart": "Carrito de Compras", "checkout": "Pagar" }

Use a library like i18next to load and display them:

const { t } = useTranslation();
return <h1>{t('greeting')}</h1>; // "Hello" or "Hola"

Important HTTP headers

Content-Language — tells the browser what language the response is in:

Content-Language: es

hreflang — tells search engines about alternate language versions (covered in detail in the multilingual site concerns question).

Best practices

  • Always provide a manual language switcher — do not rely only on auto-detection
  • Handle pluralization rules (different per language)
  • Test with actual translations, not just English in both slots

Interview Tip

Cover: language detection (Accept-Language header), URL structure (subdirectories), translation files, and SEO (hreflang). The most important point is to always let users switch manually — automatic detection is a starting point, not the final answer.

Why interviewers ask this

This tests practical knowledge of internationalization. Companies with global users need developers who can build multilingual experiences correctly, from server-side detection to client-side rendering to SEO.

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