You don't need to snoop on your visitors to choose a language. Choosing an ad based on browser preferred language or geoip is not behavioral advertising. It is behavioral advertising when you record the data, when you track the user and show your ad on a site with the lowest bid instead of the relevant site that your target audience originally visited.

You don't need the persons purchasing history, sex, marital status, age, ethnicity, interests, political orientation, a list of visited sites and all data you can get your hands on, to choose a language.