A visitor lands on your site for the first time. Their request comes from an IP address in Switzerland, their browser says it prefers French, and they've just clicked a link from a German newsletter. Which language do you show them? Which currency?
You can't know for sure. Every signal you have is a guess, and some of them contradict each other. This chapter is about collecting those signals, deciding how much to trust each one, and making it easy for the user to correct you when you get it wrong.
The most important rule comes first, because everything else follows from it. Always let the user choose their own language.