On PHPMaster.com there's a new tutorial showing how to use the localization support that comes with WordPress to make using different language definitions much simpler.
WordPress itself is translated to many languages and people can use it in their preferred language. But this is not the case with themes, plugins, and other front-end customizations. [...] The purpose of this article is to show you how to translate properly any theme or plugin by using internationalization and localization methods. In brief, internationalization (i18n) deals with making sure strings of text are wrapped in specific function calls.
He starts by introducing the PHP functions (and configuration) you'll need to get the localization support up and running in your plugin, including a few examples of how to translate a string. Also included into the post is an introduction to using the Poedit software to create the different translation files.