Fun fact of the day: about 37% of WordPress downloads are for non-English, localized versions.
So as a plugin or theme author, you should be thinking of localization and internationalization (L10N and I18N) as pretty much a fact of life by this point.
An excellent post from Otto on improvements to make and pitfalls to look out for when performing i18n on your WordPress theme.
During the process of creating my first public WordPress theme one thing was very important to me – I wanted the theme to be useful to as many people as possible. To reach this goal, I knew that I would need to make sure that my theme could be used in any language. Luckily, WordPress core provides a few different functions that makes this pretty easy to do. If this is a new topic for you, please read more about Internationalization for WordPress Developers in the codex.