Can't promise that this is THE most logical way, but this is what I did recently:
Depending on the size of your site create an englishLang.php or englishBlahLang.php (where blah is the name of the page) file and include variables for each section of content or important pieve of information (e.g. $langSiteName = "My Site" ).
When you have the whole site how you want it translate that language file as needed.
Store the selected language using sessions, and then just include the relevant file for each page.
The site I did this on did not have a great deal of text, so I just did one file per language. So far it has been pretty easy to maintain.
