1. Open "en_US.po" File From "Theme > Languages" folder using po editor software.
2. Save it as in your language, for example your language is "spain" you have to save it as "es_ES.po", to find your language code visit this website: https://make.wordpress.org/polyglots/teams/
3. Translate English Strings in to your language using google translator https://translate.google.co.in/ 
4. After Completing,Translating update file by clicking "Update" button located at top bar.

Can Also Visit The FOllowing Links To Learn More About Translating Wordpress Themes:

https://developer.wordpress.org/themes/functionality/localization/
https://developer.wordpress.org/reference/functions/load_theme_textdomain