Passing Objects between scripts and guarateeing that a language has been choosen.

Hi, I am just starting to understand how the session handling routines work in php. I can pass objects to other scripts. So I can put the language into the object that the visitor chose at the very first page. But what if a visitor bookmarked my homepage and comes back after a few weeks. How can I guarantee that a language is always choosen? I mean how is this done professionally? What do I have to write at the beginning of every page? I hope it is clear what I am asking. Thank you very much.