Before starting the installation process, check the System Requirements page concerning the prerequisites for installing LibreOffice on an Apple Macintosh system.
If you are running MacOS X 10.8 (Mountain Lion), you may run into a problem with Apple's new Gatekeeper. If so, you may find this article helpful. An easier solution to install the software and to run it the first time, right-click (control-click) the LibreOffice.app icon and choose Open from the context menu. You will be asked if you really want to start the program, confirm this to launch it.
You do not have to uninstall any previously-installed version of LibreOffice.
Download the DMG file from the Download page.
Open the downloaded DMG file by double-clicking on it in the Downloads folder, or single-clicking on the launcher in the Downloads drawer (on a standard Mac setup, it is usually on the right-hand side of the dock).
Either action opens the LibreOffice installation window. Click and drag the LibreOffice icon to the Applications folder icon.
The installation process will start. You may be required to enter your administrator password before installation proceeds (this depends on the particular user set-up, i.e. whether or not there are separate administrator and user accounts with different rights).
If you see a message requiring authentication, click the Authenticate button. In the next window, type an administrator's name and password. Click OK.
A progress indicator will appear. When the installation process completes, you will see the LibreOffice icon in the Applications folder. To start LibreOffice, double-click on this icon.
Download the DMG file for the language pack you want to install.
Double-click on the downloaded file in your Downloads folder.
Double-click on the LibreOffice Language Pack icon to start the installation process.
If no other application windows are open on the desktop, a dialog box will display a message in the language of the pack to be installed. "Welcome to the installation wizard for the LibreOffice Language Pack. This installation will update your installed versions of LibreOffice. This may take a while." (This is a translation into English of the original message which should be displayed in the language that you wish to install). If you have other application windows open, then you will notice that the Language Pack icon, visible in the dock, will jump up and down indicating that it requires attention, but is hidden from view. If you click on this icon in the dock, the welcome message will be brought to the foreground over all the other windows and become visible and you will be able to continue installation of the Language Pack.
Click on Install.
When the installation has completed, a message tells you how to change the language of the user interface from within LibreOffice.
Open LibreOffice and go to LibreOffice > Preferences > Language Settings > Languages. In the Language of User Interface list, select the required language. If you only have one language pack installed, it will already be activated so should be able to avoid this step, but it is worth checking just to be sure.
Close LibreOffice and reopen it (this should not be necessary if the default behaviour indicated above has occurred). The user interface will now be in the selected language.