this is weird
if the flexi task has a body, does it appear before or after the Arabic text?
if it appears above the Arabic text, can you check if this email firstname.lastname@example.org has a signature?
the body appears after the Arabic text, and the email email@example.com does not have a signature, why are you asking about the signature? I think this is an issue in the translation from English to Arabic in Nintex workflow.
have you seen if you can switch the language during configuring the mail templates? At Central Administration or at Site Collection level, I have a switch for the language of the template, and there I think you can select Arabic and correct the issue..
I'm currently not able to change the language at site level, but I don't know if it's an issue of my site or something general..