Air carrier pilot reported uncommanded aircraft movement after push back when the ground crew disconnected the tow bar without the brakes being set. The pilot realized the aircraft movement and immediately set the brakes and stopped the aircraft.

Narrative

During pushback from gate XX; the driver of the tug that pushed us off the gate and advised me that I was cleared to start engines. The driver then stopped the push and made no further communications. I heard noises under the plane that sounded like he was disconnecting the plane from the tug. I asked over the intercom if he was finished pushing us; and there was no response. The plane then started moving backwards and the push tug came into view. I immediately realized that the plane was rolling backwards on its own. I immediately pressed on the brakes; stopped the aircraft; and set the parking brake. I then saw the tug pull away; and the other rampers hand off the nose wheel steering pin; show it to me; and then he gave me a salute. At no time did the driver ever tell me to set the brake. At no time; did I ever tell the tug driver that the parking brake was set and he was clear to disconnect interphone. The driver of the tug did not communicate in proper English; and he did not use standard phraseology. This was an absolutely serious safety issue that could have had disastrous consequences. I called operations and asked that a message be relayed to the push crews' supervisor so that they could be immediately made aware of the issue. It is my opinion that the driver of the tug was not qualified to be doing this job. This is based on my observation of the individuals lack of proper communication and adherence to standardized operating procedures. I have seen an increase of non-standard operations from ground personnel at ZZZ recently; at it is my opinion that many of the ground crew personnel are not complying with SOPs; because they have not been properly trained; or they simply do not care to comply with the rules. I recommend that an immediate change be made before people are injured; or property is damaged.