The easiest way to translate the theme is to use Loco Translate plugin – it will allow you to create translation from the dashboard.
https://localise.biz/wordpress/plugin/beginners
You can also create manual translation by following this tutorial: https://code.tutsplus.com/tutorials/translating-your-theme--wp-25014
For manual translation I recommend free EazyPo - http://www.eazypo.ca/.
If the theme comes with the bundled plugins (Apartment WP Utils, Sepia Utils etc.) you have to translate those plugins as well.