The Samsung Galaxy A8 from last year was launched running Android 5.1 Lollipop out-of-the-box. However, recent developments indicate that the Android Marshmallow build for the device will start rolling out soon.
The dual-SIM variant of the device (SM-A800F) has received Wi-Fi Certification, and the listing clearly mentions it as running Android 6.0.
While not an official confirmation of any sorts, a Wi-Fi certification running a newer OS is usually an indication of an imminent roll-out of the update. If you own a Galaxy A8, and are eagerly waiting for try Marshmallow on your device, all we can say is that this is certainly a good omen as the update seems to be on track.