You need to set your keyboard preferences to give you two keyboards — Arabic and English: look in the configuration or settings menu.
Add a Keyboard indicator to the panel (right click on it to do that) which will show you the current keyboard and enable you to change by clicking on it.
The keyboard configuration tool will also allow you to choose a key to switch (group shift) from a list, which will have options such as both Alt keys together, Menu, Scroll lock, etc.
In OpenOffice or LibreOffice, click on Tools - Options - Language settings. There you can enable "complex text layout" and select Arabic under CTL.
If you don't like the Arabic font in Mandriva, look here: