Nisreen,
First of all, you should be aware that the default locale resolver within alfresco does rely on your browse locale for language detection, so if your browser is set to ask for en_US first for example, alfresco detects that and renders all string in English-US locale.
You can either tweak your browser setting to use arabic locale or use some browser add-on for language switching to be able to see alfresco in Arabic, and if you are using chrome, you could check out
this add-on.
If you which to switch language on a more permanent basis: Force Arabic language to be used every where in alfresco; then you should have your devs extend alfresco overriding DefaultLocaleResolver to always use Arabic for example, or maybe rely on some cookie parameter first then fallback to browser setting.
You should also manage your expectations: Arabic bundle for alfresco is not officially supported by alfresco, and even though there is and there has been a good community effort invested there, still there is a lot more to be done in order to fully support and translate all strings, support RTL script....
Regards,