Originally Posted by Stefan Blom

Just to be clear, there is a language pack (which includes proofing tools) available for Arabic.

No one here knows why there isn't a "trimmed down" version of Arabic, including only the proofing tools part of the language pack. Only Microsoft can tell.

You can post feedback for Microsoft here: http://word.uservoice.com.

Also, remember that, before Office 2013, there were no free proofing tools at all, only language packs that you could purchase.

It is still the case that most localized versions include (free) proofing tools for more than the primary language.