Your form doesn't have input id, and names fields correctly set, thus the mail never gets sent. What I think is happening is that the input fields have an English avia_ prefix at which point the Arabic suffix attempts to attach and for whatever reason that doesn't seem to work.
Here is an image of how your code looks live ( http://i.imgur.com/S1WH7.png ) and this is how the demo site looks ( http://i.imgur.com/Vfd7E.png )
What's interesting is that the user name itself is in English (the field) so the hidden field carries over without a problem. Is the WordPress backend in Arabic? I think if language is kept consistent in those fields which visitors won't see, that will solve the problem.
[Edit] Now that I took a further look, Perhaps you could look in Choices > Theme Options > Contact and see if you have a value set for the Labels field, one for each form element, because I don't see any on the form itself as I fill it out. I think those labels are actually the variable name that gets attached to the prefix. Would you set the Labels please in the Theme Options, and we can always make them not display with css.