If a theme is advertised as localization and internationalization ready, then you can easily translate the theme. Each will contain a .po and .mo files which you can use to translate the theme. Using a translation tool, such as Poedit you can generate new files with your translations.

After you generated your new .po and .mo files it’s time to tell WordPress that it should use them.

Go to your wp-config.file and add a define (‘WP_LANG’ , ‘…’ ); You can have a more detailed of what you need to insert in the wp-config.php file here : http://codex.wordpress.org/Installing_WordPress_in_Your_Language

If you do not want to use the .po and .mo files then you can use some plugins which are really good, such as QTranslate  or WPML.