If users modified their comments then flag option would be useless. Anyone could post stupid and insulting comments then modify it after a while. I wish there is another way.

@Jonathan: Thank you for clarify this for me. I am fully understood the problem. I put my self in italki moderator shoes. If I design such site then correction option can be interface with flag option. Most people would use it to correct their typos but still there are some minorities maight use it in bad way. Like vote up and down option, some people use multiple accounts to vote down some topics.  I think this is the main reason if not the only one that we can't edit our comments and I understand it. Anyway, like the others, I really want to edit my comments.

Oh come on ! are you guys new on the web ? you have never been in any other forum ?
All the forums I know till now allow to modify your posts or comments, not only in order to correct your language mistakes but also to add something if you forgot, or simply to make it more clear and understandable; You can also add details or links, or anythink.

So when you guys say "  Anyone could post stupid and insulting comments then modify it after a while. I wish there is another way "

This doesn't make sense. I have never come acorss this kind of issus. Do you really think we should restrict the freedom of people only in order to avoid potential abuses ?! This logic would be dagerous if applicated to the society ! 

You can check the text for errors and post it when you are sure it has no mistakes :)

@Steveh, yes what you propose would be a a very good improvement

@Sophie, we can only hope so....