Thank you for your question.
Could the Arabic language be supported in outlook?
If the Arabic language could not be supported in outlook, we could replace Arabic language in C:\Program Files\Microsoft\Exchange Server\Bin(By default) directory, then restart the Exchange server to check if the issue persist.
If the Arabic language could be supported in outlook, we could restart IIS and Microsoft Exchange Information Store to check if the issue persist.
We could check if there are any errors in application log.
If there are any questions regarding this issue, please be free to let me know.