Translations of the eric6 IDE are available as separate downloads. There are two ways to install them.
The first possibility is to install them together with eric6. In order to do that, simply extract the downloaded archives into the same place as the eric6 archive and follow the installation instructions of the eric6 README.
The second possibility is to install them separately. Extract the downloaded archives and execute the install-i18n.py script (type python install-i18n.py -h for some help). This way you can make the translations available to everybody or just to the user executing the installation command (if using the -p switch).