Accept-Language header
Also called: accept-language, accept-language request header
The Accept-Language header is an HTTP request header a browser sends to tell the server which natural languages and locales the user prefers, ranked by optional quality (q) weights. Servers use it for content negotiation and confirm the chosen language with a Content-Language response header.
A request carrying this header looks like:
Accept-Language: en-US, en;q=0.9, es;q=0.8, *;q=0.5Each entry is a language tag (with an optional region) plus a weight from 0 to 1; the default weight is 1 and the highest weight wins. The server reads this during content negotiation to pick a language, then confirms its choice with a Content-Language response header. MDN notes the header “serves as a hint when the server cannot determine the target content language otherwise,” and that a server “should never override an explicit user language choice.”
Why it matters for SEO
Do not gate indexable content solely on Accept-Language. Google states its crawler “sends HTTP requests without setting Accept-Language in the request header,” and Googlebot’s default IPs are US-based. A store that silently swaps language or currency off this header can show crawlers only its fallback version, hiding localized pages from search. Google’s guidance is to “use separate locale URL configurations” annotated with rel="alternate" hreflang. Google added geo-distributed crawling from non-US IPs to partly compensate, but it does not promise full coverage. Privacy trimming weakens the signal further: Safari and Chrome incognito send only one language, so treat Accept-Language as a soft default for humans, not a hard branch for machines.
How it affects your traffic
If your multilingual or multi-currency store auto-switches content on Accept-Language, Googlebot likely only ever sees the US-English fallback, so your Spanish, German, or French product pages may never get indexed or ranked in those markets. The fix is unglamorous but load-bearing: give every locale its own crawlable URL, wire up hreflang, and keep Accept-Language as a convenience redirect for humans only, never as the gate on what a crawler can reach. That URL-plus-hreflang architecture, built so each market's pages actually earn rankings, is the backbone of our e-commerce SEO work.
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