After doing very extensive searching, I am sorry to report that I could not find any English translations of any of Agota Kristof's plays.
On a side note, Agota Kristof was Hungarian. She lived in Switzerland (having left Hungary given the Russian invasion in 1956) and wrote in French. She was born on October 30, 1935 and died (at her home in Switzerland) on July 27, 2011.
Agota wrote, and published, the following plays:
John et Joe (John and Joe (1972))
Un rat qui passe (A Rat Passes(1972))
La Clé de l'ascenseur (The Key to the Elevator (1977))
L'Heure grise ou le dernier client (The Grey Hour or The Last Customer (1975))
Le Monstre : et autres pièces (The Monster: and other parts (2007))
While none of her plays were translated into English, three of her novels have been. The following novels were translated into English:
The Notebook, translated by Alan Sheridan (Le grand cahier, 1986)
The Proof, translated by David Watson (La preuve, 1988)
The Third Lie, translated by Marc Romano (Le troisième mensonge, 1991)