Able to switch between different langauges

Hello,

I want to give people the option to swich between 3 languages. I would like to add this in the right top corner in the header. I thought I would have to do it via the file header.php but this is the same for every page. So the problem is then that I do not know which code to enter so that when you are on a certain page the language just switches but not the page.