Escaping Montevideo Harbour – possibly. Getting all the way back to Germany – extremely doubtful.
In terms of getting out of the harbour, Langsdorff would have had two options: heading across the river Plate to Argentina, or making a break for the open sea. If he’d headed for Argentina, there was no guarantee that he’d be any better off than in Uruguay. Argentina had some Axis sympathies, I think, but it was still nominally neutral, and the British and French would have put the same diplomatic pressures on it as they had put on Uruguay to respect the Geneva Convention and either intern the ship or force it to leave. Making a break for the open sea was problematic too. The British had made sure that the Graf Spee’s departure from Montevideo was in a carefully defined time window by making use of an article in the Geneva Convention that prohibited a warship from sailing from a neutral harbor less than (I think) 24 hours after an enemy merchantman had left the same harbour. There were British merchant ships in Montevideo at the time of the Spee incident, and the British arranged for one of them to leave the harbour at the right moment to put Langsdorff in a bind between the Geneva Convention and the expiration of the Uruguayan government’s deadline for the Spee to leave. Harwood’s cruisers were lying in wait nearby, and it’s doubtful that the Spee could have given them the slip. Moreover, the shallow waters of the Rio de la Plata area were unsuited for combat from Langsdorff’s perspective because his water intakes might have gotten clogged with mud. Additionally, having the ship sink in combat in shallow water carried the risk that secret equipment and papers would have fallen into Allied hands (this being less of a concern if, as eventually happened, the ship was scuttled under controlled conditions).
Assuming for the sake of argument, however, that Langsdorff had managed to elude Harwood, getting back to Germany would have been very challenging. Although the Spee was technically still able to sail and fight, the damage she had suffered in the original battle with Harwood had left her in very poor condition to make a long sea voyage. Harwood, even if he’d initially been eluded, would still have been close by and would have sent his ships (and his scout plane) to look for the Spee; additionally, some British and French naval groups were already converging on the general area following Harwood’s initial battle a few days earlier, so Langsdorff would have had an awful lot of sea power focused on finding him.