Once Arabic for Confluence is installed a new, Arabic language, will be available in the list of languages.
First click on your Avatar and select "Profile"
Then when you are redirected to your profile page, Select the "Settings" tab and click on the "Edit" button.
Doing so, you will get a pop-up screen where you can select your language and you can choose "Arabic" from the menu.
And Voila! Your Confluence will be translated to Arabic language