What are Content-Language and Accept-Language?
Content-Language is the language of the page you're serving. Accept-Language is a list of languages you PREFER to accept.
Content-Language, an entity header, is used to describe the language(s) intended for the audience, so that it allows a user to differentiate according to the users' own preferred language. Entity headers are used in both, HTTP requests and responses.1
Accept-Language, a request HTTP header, advertises which languages the client is able to understand, and which locale variant is preferred.2 There can be multiple languages, each with an optional weight or 'quality' value. For example:
Accept-Language: da, en-GB;q=0.8, en;q=0.7
(The default weight is 1, so this is equivalent to da;q=1, en-GB;q=0.8, en;q=0.7).
You're going to have to parse the values and weights to see if an appropriate translation is available, and provide the user the translation in the highest preferred language weight.
It is recommended you give the users an alternative, such as a cookie set value, to force a certain language for your site. This is because some users may want to see your site in a certain language, without changing their language acceptance preferences.