PrestaShop 1.7. How to add a new language

1. In your PrestaShop admin panel go to International -> Translations section: 
2. Find Add/Update a language block. Select a language you want to add from the dropdown list. Click the Add or update a language button: 
3. A  new language has been added successfully. Check your website to see the changes.

Hopefully, this tutorial was helpful for you.