I took the current www.wikipedia.org page and added an auto-selection based on the browser language settings. This example recognizes all the languages with 10.000+ articles:
http://www.tommasoconforti.com/portal2.php
Try to set your browser to Russian or Esperanto and see what happens :-) If the language preference is not recognized, no selection appear.
It's basically the same page at www.wikipedia.org, with a few lines of php inserted. I copied over wikimedia's stylesheets and images since hitting wikipedia servers was too slow. Japanese and Chinese characters might be incorrect since I don't have them installed and I have no idea of what I put on the page :-))
Source on http://meta.wikimedia.org/wiki/Portal_code
Alfio