For translating a WordPress theme you need to create translation file with the file extension of .po from our [themename].pot file located in themes/themename/languages/
For example, if you are translating the theme to french you need to create translation file (fr-FR.po) and put the file in to wp-content/themes/themename/languages/fr-FR.po
For more language codes please look at http://www.lingoes.net/en/translator/langcode.htm
You can use poedit software https://poedit.net/ which will make the job easy. Please follow the below tutorials for more information about translating a WordPress theme
https://www.hostinger.com/tutorials/wordpress/how-to-translate-wordpress-theme-using-poedit