How do you serve content in multiple languages?

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

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.