I have learnt from selling WordPress plugins that the WordPress user-base is more than just us english-speakers, the demand for translatable themes and plugins is huge. If you are going to be releasing a theme publicly, then you should take localization into account. Translations are carried about by using some special code in your theme where the translatable text is and by providing .po files. The .po files contain the actual translations and you can create one for any language. This is a roundup post with some sources which cover the subject in detail so you can get started easily.
Internationalization and localization are terms used to describe the effort to make WordPress (and other such projects) available in languages other than English, for people from different locales, who use different dialects and local preferences.
How to make a translatable WordPress theme
In this step-by-step tutorial, you’ll learn how to take a WordPress theme and make it translatable for any language.
Translating WordPress Plugins & Themes
This guide is another in the Inside WordPress series and attempts to show how to produce a translation for an already-prepared theme, plugin, or WordPress itself.
How to translate a WordPress theme
If you are familiar with WordPress and the many plugins and themes available for it, you’ve probably come across some strangely named files like .mo, .po, and .pot. If you haven’t then the theme and/or plugin has not been designed for translation. This article explains how to take a .po file that is included with your WordPress theme and/or plugin download and translate it into your native language.
WordPress theme localization
In order to run multilingual WordPress sites both contents and the theme need to be localized. In this post, I’ll describe what we did in order to fully localize our own theme for wpml.org.
If you would like to include translations with your theme or plugin, a very usefull service is OneHourTranslations - you can upload a text file and specify which languages to translate to and from and will get it done professionally from .5cents a word.