I would suggest you subtitle exactly as your subject speaks. Documentary films are supposed to be accurate, representing truth. If a deaf person learns that the subtitles were not reflecting actually what was said then you lose credibility. Also, I don't think your subject would appreciate words being put in their mouth.
If your subject is really that hard to understand, then can you not include them in your film? I always vet my subjects for camera presence before I spend the money and time with them.
Ramona, that's a shame you don't want to subtitle your films. I'm hard of hearing to point where I need to wear $6,000 hearing aids. I absolutely must have subtitles for me to watch a film and understand most of it. Then there are people who are completely deaf and they need subtitling too.