If you have an older translation version already installed you need to remove previous pack and upload a new one.
In overview, the process for updating a Language Pack is
Uninstall the old language pack
Run a Quick Repair and Rebuild
Install the new language pack
Run a Quick Repair and Rebuild
Login as English
Go to Admin / Module Loader
Uninstall previous language pack by clicking Uninstall followed by Commit
Go to Admin / Repair / Quick Repair and Rebuild
Logout
Just follow the normal installation steps.
Content is available under GNU Free Documentation License 1.3 or later unless otherwise noted.