How to check accept language header

Send the header yourself and read Content-Language on the reply. Run curl -sS -D - -H 'Accept-Language: fr;q=0.9, de' http://127.0.0.1:8391/. The answer is German, because a tag with no q carries a quality of 1 and outranks the French one. Then test a wildcard, a refusal and a region subtag.

Why check this

Language selection is the one piece of content negotiation a tester cannot drive from the browser address bar, so it tends to reach production tested in exactly one configuration: the machine of the person who wrote it. Run this check when the product gains a second language, and again after any change to the routing or caching layer in front of it.

The failure it prevents is a customer who sets Portuguese as a preference and is served English, while the log shows the header arrived correctly. The header did arrive. The parser dropped it.

Prerequisites

• curl 8.21.0. The curl manual for -H covers sending a request header.
• Node 22 and a free port. Save this as neg.mjs and run node neg.mjs. It is the target for steps 1 to 3, and it follows RFC 9110 section 12.5.4.

import { createServer } from 'node:http';

const page = { en: 'Cart', de: 'Warenkorb', fr: 'Panier', pt: 'Carrinho' };

function parse(header) {
  return (header ?? '')
    .split(',')
    .map((part) => {
      const [tag, ...params] = part.trim().split(';');
      const q = params.map((p) => p.trim()).find((p) => p.startsWith('q='));
      return { tag: tag.toLowerCase(), q: q === undefined ? 1 : Number(q.slice(2)) };
    })
    .filter((e) => e.tag !== '' && Number.isFinite(e.q))
    .sort((a, b) => b.q - a.q);
}

function negotiate(header) {
  const entries = parse(header);
  const refused = new Set(entries.filter((e) => e.q === 0).map((e) => e.tag.split('-')[0]));
  for (const { tag, q } of entries) {
    if (q === 0) continue;
    if (tag === '*') {
      const any = Object.keys(page).find((l) => !refused.has(l));
      if (any !== undefined) return any;
      continue;
    }
    if (page[tag] !== undefined) return tag;
    const base = tag.split('-')[0];
    if (page[base] !== undefined) return base;
  }
  return 'en';
}

createServer((req, res) => {
  const header = req.headers['accept-language'];
  const locale = negotiate(header);
  res.writeHead(200, {
    'Content-Type': 'text/plain; charset=utf-8',
    'Content-Language': locale,
    Vary: 'Accept-Language',
  });
  res.end(
    'sent: ' + (header ?? '(absent)') +
    '\nparsed: ' + JSON.stringify(parse(header)) +
    '\nserved: ' + locale + ' "' + page[locale] + '"\n'
  );
}).listen(8391, '127.0.0.1', () => console.log('listening on 8391'));

• Stop it when you are done. On Windows, find the PID with netstat -ano | grep 8391 and run powershell -Command "Stop-Process -Id <pid> -Force".

Steps

1. 1

   Step 1.

   Send two languages where the second one carries no q parameter.

   curl -sS -D - -H 'Accept-Language: fr;q=0.9, de' http://127.0.0.1:8391/
   

   HTTP/1.1 200 OK
   Content-Type: text/plain; charset=utf-8
   Content-Language: de
   Vary: Accept-Language
   Date: Fri, 11 Sep 2026 22:46:43 GMT
   Connection: keep-alive
   Keep-Alive: timeout=5
   Transfer-Encoding: chunked
   
   sent: fr;q=0.9, de
   parsed: [{"tag":"de","q":1},{"tag":"fr","q":0.9}]
   served: de "Warenkorb"

   German wins from second place. An absent q means 1, and 1 beats 0.9. Position in the header is not preference order.

2. 2

   Step 2.

   Ask for a region that has no translation of its own.

   curl -sS -H 'Accept-Language: pt-BR' http://127.0.0.1:8391/
   

   sent: pt-BR
   parsed: [{"tag":"pt-br","q":1}]
   served: pt "Carrinho"

   The catalogue holds pt and the request asked for pt-BR. Truncating the tag to its primary subtag is what keeps Brazilian visitors out of the English page.

3. 3

   Step 3.

   Refuse one language and accept anything else.

   curl -sS -H 'Accept-Language: en;q=0, *' http://127.0.0.1:8391/
   

   sent: en;q=0, *
   parsed: [{"tag":"*","q":1},{"tag":"en","q":0}]
   served: de "Warenkorb"

   q=0 means not acceptable, and * matches every remaining tag. English is the default of this server and it is still the one language it may not serve here.

4. 4

   Step 4.

   Run the parser most services actually have against the same three headers. Save it as naive.mjs.

   const headers = ['fr;q=0.9, de', 'pt-BR', 'en;q=0, *'];
   const naive = (h) => h.split(',')[0].split(';')[0].trim();
   for (const h of headers) console.log(h.padEnd(14) + ' naive: ' + naive(h));
   

   fr;q=0.9, de   naive: fr
   pt-BR          naive: pt-BR
   en;q=0, *      naive: en

   Three headers, three wrong answers: French instead of German, a tag with no catalogue entry, and English for a client that refused English.

5. 5

   Step 5.

   Watch a public site negotiate, and read its Vary in the same response.

   curl -sS -I -H 'Accept-Language: de,en;q=0.5' https://developer.mozilla.org/
   

   HTTP/2 302
   location: /de/
   vary: Accept

   The redirect target came from the header. The Vary list does not mention it.

6. 6

   Step 6.

   Repeat the request with an English preference and compare.

   curl -sS -I -H 'Accept-Language: en-US,en;q=0.9' https://developer.mozilla.org/
   

   HTTP/2 302
   location: /en-US/
   vary: Accept

   One URL, two Location values, decided by a header the response never declares as varying.

How to read the result

| What you see | What it means | What to do | | --- | --- | --- | | Content-Language matches the highest q tag you sent | The parser sorts by quality | Repeat with the q values reversed to confirm it sorts rather than guesses. | | The first tag in the header always wins | The parser reads position, not q | File it. A browser sends its second choice with q=0.9 and its first with none. | | A pt-BR request returns the default | There is no fallback to the primary subtag | Add tag truncation before the default. Most catalogues hold pt, not pt-BR. | | A q=0 language comes back anyway | q=0 is read as absent, or as a low score | Treat 0 as a refusal and drop that tag from the candidate list. | | Vary does not list Accept-Language | A shared cache may serve one language to everyone | Add it, then confirm with How to check vary header. |

Common mistakes

What to check next

• How to check the browser locale: where the header the browser sends comes from, and why it differs from navigator.language.
• How to change locale in chrome: drive the same negotiation from a browser instead of curl.
• How to check vary header: the caching half of the pitfall above.
• How to check for missing translations: what happens once negotiation picks a language the catalogue only half covers.
• How to check hreflang tags: the other way a site declares which language a URL serves.

FAQ

How do I test Accept-Language without changing my browser?

Send it with curl, as in every step above. The header is a plain request header, so any HTTP client can set it, and no browser setting is touched.

Does a missing q value mean zero?

It means 1, the highest quality there is. Tags rank by q and not by position, so a tag with no q outranks every tag that carries one.

What should the server do when nothing matches?

Serve a documented default and name it in Content-Language. Returning 406 for language is allowed by RFC 9110 and shows the visitor nothing useful in any browser.

Does Content-Language change the page language for search engines?

It declares the language of the body that was sent. Crawlers mostly follow the URL and the hreflang links instead, which is a separate check.

Why does a proxy break negotiation?

A cache that stores one copy per URL serves the first language it saw to everyone after that. The response has to carry Vary: Accept-Language for the cache to key on the header.

Verified