The process of adapting a program to foreign languages is called internationalization and localization, or i18n and L10n for short.
Internationalization is the first step, where developers choose a mechanism for the translation of the software. All the strings of the software are then converted to make use of this mechanism.
Localization is the second step, where bi- or multi-lingual users of the software actually translate the original strings into their own language. Localization may also include providing local data in order to further improve the software for local users.
You can learn more about i18n and L10n at this Wikipedia page: http://en.wikipedia.org/wiki/Internationalization_and_localization
The PrestaShop team has chosen to use its own built-in translation tool, so that anyone with a PrestaShop installation may customize their shop to their needs.
The community translation is done with an online and open installation of PrestaShoptool, located at httpat https://translatecrowdin.net/project/prestashop-official.com/.
The official translation packs can be downloaded manually from this address: http://www.prestashop.com/en/translations.