language negotiation

When a browser requests a page, it sends an Accept-Language header listing preferred languages with quality weights, for example Accept-Language: es-ES,es;q=0.9,en;q=0.7. The server reads that list, matches it against the languages it has, serves the best fit, and labels the response with a Content-Language header. Any response that varies by this header should also send Vary: Accept-Language so caches and CDNs store each language separately.

The SEO catch

Two patterns cause the most damage. First, auto-redirecting visitors to a language version based on the header. Google advises against this because it can stop users and crawlers from reaching your other versions. Second, serving different content on the same URL depending on the header, which hides every other language behind one address.

Googlebot mostly crawls from US IP addresses and, by default, sends requests without an Accept-Language header. So if your store decides language purely from that header, Google usually sees only your fallback language, and the rest may never get indexed. The fix Google recommends: give each language its own URL, connect them with hreflang annotations, and offer a visible language menu instead of a forced redirect. Keep negotiation as a hint for first-time visitors, not a wall that traps them (or a crawler) in one locale.