In order to have a multilingual website, you need to install one of these two plugins:
The theme is fully compatible with both of them and you can use any of them. Each plugin has some advantages and disadvantages. Hence, you can visit their author’s website and choose one of them depends on your needs.
Also, both of them have strong documentation that you can use them to build your multilingual website.
Here, is WPML documentation. Also, you can find Polylang documentation here.
Regarding the translating widgets with WPML check Translating Widgets
Additionally, don’t forget to keep your theme up to date always to have the latest version of the .pot file.