If you are running LibreOffice under linux, you can install from the repositories, either using the appropiate sofware or from the command line, e.g.
sudo apt-get install libreoffice-l10n-ar libreoffice-help-ar
The actual command will depend on your particular distribution, but the package names are most likely those shown here.
If you have installed your LO from packages downloaded from libreoffice.org, go to https://www.libreoffice.org/download/?nodetect; select
.rpm and your architecture, click on the link for Arabic and download the translated user interface and the help pack.
Install the usual way for these cases, for example,
.deb files can be installed with,
sudo dpkg -i package-name.deb
If you are using Windows, you can run the LO installer and follow these instructions (see also the Windows installation instructions).
What you do is to select “Custom” installation instead of “Typical”, and then add Arabic under “Additional user interface languages”.