Air Traffic Controller reported aircraft descended out of their assigned altitude resulting in the aircraft flying below the MVA and towards terrain. The aircraft did not respond to ATC instructions until communications was reestablished and aircraft was issued a climb that they acknowledged.

Narrative

Aircraft X departed ZZZ on runway heading climbing to 030. I handed the aircraft off to ZZZ Departure so they could climb them out the south gate ZZZZZ. They accepted the handoff and I issued the frequency change to the aircraft which they read back. I continued working my traffic. I noticed shortly thereafter that their altitude had not climbed out of 030 but instead was beginning to descend. I reach out to them and the briefly responded and I verified they were climbing back to 030. They did not respond. I checked with the Departure Controller who could not get a hold of them either. I tried them on guard with no response. They began descending near the MVA at which time I issued a Low Altitude Alert. I then reached out to ZZZ Tower to see if they had contact with the aircraft. They continued descending and got as low as 014 in a 024 MVA area.Shortly after the aircraft responded to my calls. They sounded confused and there appeared to be a language barrier. I reissued a climb and got them on course. When I questioned the pilot why they had descended he did not seem to understand what I was asking. I handed them off to ZZZ Center.Recommendation - I don't have a specific recommendation for this; just would like to know what happened during that time in the aircraft.