You need to activate Arabic language as below:

1. You need to login with admin user.
2. Enable developer mode by going to settings and click on activate developer mode.
3. Go to settings and from Translations menu click on Languages menu.
4. You will get all languages then click on activate button in the record of Arabic/العربية.
5. The Arabic language will be activated, and you will get a confirmation popup message and you can click on "Switch to Arabic/العربية & close" button.
6. Page will be refreshed and open with Arabic Interface.

If the interface opened and the interface is not right-to-left, you need to install the  rtlcss package:

To install rtlcss package if you installed Odoo on Windows OS, Follow the below steps:

1. Download and install nodejs.
2. Install rtlcss with the below command in windows command prompt or power shell:

npm install -g rtlcss

3. Edit the System Environment’s variable PATH to add the folder where rtlcss.cmd is located (typically: C:\Users\\AppData\Roaming\npm\)

If you installed Odoo on Ubuntu OS, you could install rtlcss as below:

1. Download and install nodejs and npm with your package manager.

2. Install rtlcss:

   $ sudo npm install -g rtlcss