One person from Iran write a question and there is no answer to it.
If we discuss in Persian may be I can understand better his problem and answer it.
Can I do it generally in comments, with Persian people?
I prefer the comments to be in English. Note that math.SE is not just about question getting answered, but helping other users who are browsing that question, looking for answers. While use of Persian in the comments might make your communication easier with the OP and clarify a great deal to the OP, there are users who don't know Persian reading that post, and perhaps they'd miss some important clarification done in the comments (in Persian, say).
What you can do, on the other hand, is to post all the clarifications in English and discuss about them (in Persian, say) on the chat. This wouldn't confuse readers of the post and you'd be able to clarify OP's doubts in their language too.