There are several layers involved in the processes that provide internationalization capabilities to Plone. Basically they are divided in the ones responsible to translate the user interface and the display of the localization particularities (dates, etc):
Adapting locale-specific settings (such as the time format) for the site, like the plone.i18n package.
And the ones responsible for translating the user generated content. Since Plone 5, this is done out of the box with plone.app.multilingual
plone.app.multilingual (Archetypes and Dexterity content types, requires at least Plone 4.1)