A full header might look like this: Accept-Language: en-US,en;q=0.9,fr-FR;q=0.8 This tells the server: "I prefer US English (1.0), but I'll take any English (0.9) or French from France (0.8) if that's what you have".
Are you trying to or set up language detection for a specific web application? The role of Accept-Language header in software localization 0.9,fr-FR
When a server receives this header, it attempts to match the request with available content: A full header might look like this: Accept-Language: