Not everybody blogs in English, and neither should your WordPress theme be exclusively coded that way. With the help of localization plugins you can ensure that the next WordPress theme you’re building has text that can be translated into different languages. Cats Who Code has a great tutorial on it.

The gist is: Whenever you’re writing custom text, be sure to enclose it in the _e('text, 'domain') and __('text', 'domain') functions. Both basically output text as echo normally would, but used with a language file editor it lets you translate text quickly and easily. _e() is for printing text the usual way while __() is for printing text inside of PHP functions.

For more details, check out How to make a translatable WordPress theme.