New from the CatsWhoCode.com blog there's a few helpful hints on making your WordPress theme easily translatable:

Althought English is the most represented language over the Internet, it is a good thing to think about people who speak other languages and offer them trabslated WordPress theme. In this step-by-step tutorial, you'll learn how to take a WordPress theme and make it translatable for any language.

You'll need to add in a few functions to handle the loading of various language templates and use them, along with language-specific .PO files, to make switching languages as simple as changing the value of a constant in a define.