Belgian Malinois are definitely not for the feint of heart. They can be a handful, and when kept for protection, they can often inadvertently be trained to be too protective, leading to surrender at shelters or euthanize for aggression.
It's difficult to know whether an individual dog will attack and kill chickens. With proper training, most dogs can be taught to avoid chickens, but many can never be trusted to be alone with them.
If you already have outside dogs that will make noise when strangers enter the property, that is enough to deter many thieves. Especially if you have more than one dog. Thieves will often avoid situations that may call attention to themselves, and that includes barking dogs.
If you really think you need extra protection, installing a security system may be a good option that will be immediate and take less time. This breed of dog requires a knowledgeable person to train them (of course I'm not sure of your expertise in the subject so you may be more than capable) and they also require extensive training. They are also fairly high strung dogs that will require regular exercise and stimulation, especially if kept in a house.
If you have valuables you are worried about now, and you happen to have a attic, put all the stuff you don't want stolen up there. My dad did 25 years on a police force handling burglaries and he said never once did anyone steal things from an attic!
Eta: I'm on my iPhone, so please excuse any weird spelling errors!
