Are your themes translatable?
Yes, translation/localization is very easy.
Multilingual
If you want to run a multilingual site I highly recommend to use the most popular WPML multilingual plugin.
The Realty theme for example is officially certified by WPML, see http://wpml.org/theme/realty
Single language
If you want to translate the theme from english into another language I recommend using the free POedit software.
But first open wp-config.php, which is located in the root directory of your WordPress installation, and update the following line from:
define('WPLANG', '');
to
define('WPLANG', 'de_DE');
The example above sets your default language to german. You can set any language you want. Instead of using “de_DE” you insert the language code of your specific language.
For a complete list of language codes, see column “WordPress Locale”: http://wpcentral.io/internationalization/
How to translate strings
Within the theme folder is folder named “languages”, that contains all localization files. The default language file is “default.po”. Copy and rename it to the language you have set in wp-config.php. If you want to translate into Spanish you would have to name it “es_ES.po”
Now download and install the free POedit software and open your new PO file. Then you translate all your desired strings into your target language, save your changes, and refresh your site. Your translated strings should now be displayed instead of the default english strings.