On this page you can get a link for your language that allows you to download files that turn your wordpress installation (the non theme files) into your desired language. You need the .mo and .po files for your language (Example link for german version here)
There core wordpress .mo and .po files you downloaded go into your wp-content/languages folder.
In the root of your wordpress installation you will find the wp-config.php file. Open it and change define (‘WPLANG’, ”); to reflect your country language code, e.g.: define (‘WPLANG’, ‘de_DE’);.
You will find the theme when you scroll down. You can customize the available languages (Deutsch/English) or add your own language.
After adding a new language, hit “Rescan” so the plugin can go through the theme files and get all the custom language fields.
Clicking “Edit” you can then translate those language fields.