I have the same problem. Reported it to Zimbra and its not a problem with the MSI. Its a problem with many multi language MSIs. They sent me a command to manually change the MSI to English only. When I didn't do that it does not cause problems for my users when its set as Arabic. Outlook works just fine. Its just when uninstalling or repairing the software its in Arabic.
I know its not a solution, but Zimbra looked into it and did not see a problem with the MSI. Search around the internet and you will see many MSIs have this problem and the "fix" is to just set the language of the MSI manually.
But feel free to correct me if I am wrong! I would love to see an actual fix, not a work around.
PS: This has been a "problem" since they released multi language MSIs for ZCO. Which I believe was v7+. Also here is the bug report I had open on this:
https://bugzilla.zimbra.com/show_bug.cgi?id=58719#c4