Login to your prestashop store admin panel then navigate to International > Translation section and 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. A  new language has been added successfully.

Setup default language for your store

Login to your PrestaShop store admin panel and navigate to International -> Localization section. In Localization tab find the Configuration section. For the Default language option choose the language you want to be used by default.