Poedit is a common program which you can use to translate the theme. It is available for free on poedit.net.
In this tutorial, I want to show how to translate your WordPress site use PoEdit plugin.
After you have installed Poedit, you can open it and select File > New Catalog from POT file. Then select the .pot file from the theme you wish to translate which you can find in the /includes/lang/ or /languages/ folder of your theme.
Then go through every line of text and translate it to your native language. Afterwards you can save the catalog with your language code (for example es_ES.po). Normally Poedit automatically creates the .mo file and saves it, too.
The last step you have to do is to upload your created language files to the language folder of your theme using FTP