When a user clicks on one of the flags, set a cookie to indicate which language they prefer. Then build 8 nav bars (one for each language). When the page is displayed, check the cookie and display the appropriate nav bar with an include file.
As for the page's content, databasing it is usually the easiest way. Make a table with three columns. Page # (int), Language (int 1-8), Content (text or blob or something big). Then, on each page of your site, you check to see which language the cookie is set to and you do a query in the database for the content for that page number and that language.
You will probably want to build a Add / Edit / Delete admin tool for the pages of your site so that you can easily alter the body of each page in the database.