Accept-Language: language negotiation and its limits

The header is an ordered preference list, not an instruction. A visitor sending en-GB, en;q=0.9, fr;q=0.8 would like British English, will accept any English, and can read French at a push. A server should walk that list in order, falling back from a region specific tag to its base language, and only then to its own default.

Language is not location. A traveller, an expatriate or anyone using a borrowed device sends a language that says nothing about where they are or what currency they hold, and browsers do not send a country at all. Using this header to pick a price, a tax rate or a shipping region produces confident, wrong answers.

Automatic redirection based on the header is the classic mistake. It makes a shared link open differently for different people, hides content from search engines that crawl with one language, and traps anyone whose device language is not the language they read. Negotiate the default, show a visible switch, and remember the explicit choice.

Because the list can be fairly distinctive, browsers have been reducing what they send: many now transmit only the primary language, or a shortened list, in a deliberate move to cut the fingerprinting surface. Design your matching so that a single tag with no region still resolves to a sensible page.