Trying to find this film. Set in 1920’s to 1940’s I think.
From memory the man had cerebral palsy or something similar. Couldn’t walk and his speech was incomprehensible to everyone. He was thought to be mentally as well as physically handicapped until years later (in his 40’s) he was in a home for handicapped people and one patient could understand his speech and translated his talking to the doctors who now realised he was very intelligent and articulate. He had been trapped in his body for years and years.
I am sure this was based on a true story and he wrote it.