Turn it over to the police if it is stolen. If you cannot see that it is stolen, then no matter what, you will not be able to activate the device without the original Apple ID and password. The phone will be useless to you. If the seller will not provide you with the Apple ID and password, then you are out of luck. If you purchased this on eBay, I would still see if you can file a claim.
The message you posted has something to do with Lost Mode. If you try and translate the exact message, you may be able to find the owner, but at least you know it is somewhere where they speak French.
Message was edited by: ChrisJ4203