http://mozdev.i2p/en-US/docs/Web/HTTP/Headers/Accept-Language
Some browsers (Chrome and Safari) add language-only fallback tags in Accept-Language —for example, en-US,en;q=0.9,zh-CN;q=0.8,zh;q=0.7 when navigator.languages is ["en-US", "zh-CN"] . For privacy purposes (reducing fingerprinting ), both Accept-Language and navigator.languages may not include the full list of user preferences, such as in Safari (always) and Chrome's incognito mode, where only one language is listed.