Can be in the item description, or in the content that you put in the item preview page.
Or maybe in the comments you are writing in the message box for the reviewer on CodeCanyon, when you submit your item.
This is an international community and English is the language most people (authors and buyers) understand, so I guess from here the language requirement.
Message to the Reviewer is
Let me know if you have any question about the code and I would Love to answer
Thanks and I Look forward to hearing back from you soon .