Poedit is a common program which you can use to translate the theme. It’s available for free on poedit.net.
After you have installed Poedit, you can open it and select File > Open. Then select the butter.pot file from the theme you wish to translate which you can find in the /languages/ folder of Butter theme.
Then go through every line of text and translate it to your native language. Afterwards you can save as the catalog with your language code (for example es_ES.po) File > Save as . 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 /languages folder of Butter theme using FTP.