How do the multilingual functionalities work so you can make the best configuration choices for your site's particular needs?
A very useful hands-on practical guide for configuring your D7 site can be found in the handbook "Drupal 7 Multilingual Sites" by Kristen Pol (Pakt Publishing, April 2012). Highly recommended, clearly written, like it a lot.
Gabor Hojtsy, the Drupal lead for all things multilingual has also blogged extensively about the models and intentions behind the various functionalities. See Gabor Hojtsy's Multilingual Drupal 7 for more.