Years ago when I was young and naive (since then I've just grown older, I suppose) I made a website that used sessions for navigation. A user would click a top level menu entry, the page would reload and present the next level menu, and so on, and each previous menu choice would be stored in a php session variable so that the correct submenu or sub-submenu would be served up.
Needless to say, this proved a bad idea. Saving deeplinks into any page was impossible because the navigation history stored in the session could not be bookmarked, and search engine spiders didn't make it past the home page since they didn't grok my weird and wonderful navigation system.
Now, though, I am concerned that I might be repeating my mistake. I'm building a website with multiple language support, and I use a session variable ($SESSION['lang'] to be precise) to store the user's language preference. If $SESSION['lang'] is not set I revert to a default language.
Will search engines be able to index all languages supported, or only the default language?