User Configuration > Administrative Templates > Microsoft Office 2016 > Tools | Options | Spelling
User
HKCU\software\policies\microsoft\office\16.0\word\options\vpref
Specifies the spelling rules to use for checking spelling of Arabic text. This option is available only if you are using a right-to-left language version of Microsoft Office or have installed Microsoft Office 2016 proofing tools or the Microsoft Office Single Language Pack 2016 for the language, and have enabled support for the language through Microsoft Office 2016 Language Preferences.