[development] Is there a guide to working with languages in D7 ?

After working in D7 for a few months now, I have to admit to still being a little confused about when to use which language. Is there a write-up somewhere that can define some best practice ? I would like to make sure that I develop my site to work properly in multiple languages, but, the language system still feels like a blur. Anyway, this is just a little probe in the dark to see if someone else has done the hard work..... Peter