An air carrier Captain reported the aircraft rolled uncommanded during pushback when the ground crew disconnected the tug before flight crew set brakes.

Narrative

Established communications with push crew on wireless head set. The driver had a very strong accent. We requested push via ACARS (PR) and received clearance 'to push tail south'.. I said to the driver; brakes released; cleared to push tail south. He said 'Roger'. On the push; he asked; 'what way do you want tail' and it was very hard to understand with a strong accent and outside noise. I repeated; 'tail south' and he responded 'Roger; cleared to start engine'.. We started #2 on the push back. On the tailing of the aircraft; I heard 2 bangs/pops but the push continued and start of the engine. While still moving and lots of background noise (aircraft engine running); I thought I heard 'set brake'; again we were still moving. I queried and asked if he wanted me to set brakes (no response). We stopped moving and again; I asked if he wanted me to set brakes. Again; no response. I queried 1 more time while looking at the marshal at my 1200 position and still no response. Next; I see the tug pulling away and the aircraft moving forward to the 1200 marshal as a breakaway. He placed the wands between his knees and signaled me to set brakes. I did and the aircraft stopped. And I signaled brakes set. The tug drove off and the push crew showed that they had the by pass/disconnect pin with red ribbon; turned and walked to the terminal. There was no communications of this or to disconnect headsets. I received a salute signal; I gave the thumbs down and asked to hook up the headsets. They attempted to but the wireless headset didn't work; or no communications. A supervisor brought another headset and communications were established. I queried WHY we rolled; was it from a sheared tow pin? He responded in a very strong accent that I could not understand. I asked again; he responded :we have pin'. I said 'that's not what I'm asking; I heard 2 pops; did we shear the push pin?' He said 'I don't know what you are asking; but brakes weren't set and you rolled; breakaway.' He asked if I wanted to talk with his supervisor. The supervisor got onto the headset. MUCH better English/ no accent; and I explained all that happened. He was able to understand my questions; broken shear pin and if I needed to call Maintenance? He checked the tug; no sheared pin and the nose with no damage. I asked if I need to call Maintenance; he said if I want to; but again; the shear pin didn't break (he personally looked at it); the crew disconnected the tug without brakes set. I was comfortable with that and we disconnected the headset. The supervisor did say he would talk with the push crew. The aircraft steered normally out to the runway.