Theme use PO translation file which is the industry standard translation method. In the theme folder, you will find a folder called languages. You will find each languages files store in the folder.

• *.po – A lists of all text strings in theme. The text strings are in English and you can add the translation to each text string.
• *.mo – This is the compiled .po file and is used by WordPress to translate the theme

If your language isn’t included in the theme language files. You have to create a new .po file from the original Theme.po located in theme’s languages folder then save the new .po file with your language code for example de_DE.po