Messages bug disappears in iOS 8.4
The reason why this specific string of characters forces iPhones to crash has to do with the notification system employed by Apple. Apparently the string of Arabic letters could not be recognized and separated by iOS. This prevented the thread from showing up in abbreviated form as other notifications are, and led to the rebooting and crashing problems.
The iOS 8.4 update is expected by the end of the month. While you wait for it to arrive, you can use the official workaround disseminated by Apple.
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source: HDBlog.it (translated)