You can easily add more languages to your PrestaShop website. To do that you have to login to your website's admin panel and go to the International > Localization menu.
Then choose the desired language in the Import a localization pack section. You may also choose what content you want to import along with the language.
By default - States, Taxes, Currencies, Languages and Units would be ticked. You may adjust those to your preferences. You should also choose if you want to download the pack data from the PrestaShop website or use the one in the localization folder of your PrestaShop installation. Once you select the preferred options, click on the Import button.
Once the pack is installed, you would see a Localization pack imported successfully message.
If you want to manage the currently installed languages, just go to the International > Translations menu. You can review, update, export languages from it and also modify translations.