What do we call dementia-related communication difficulties?

Depends on a stage: Depending on a stage and areas of the brain affected at the moment, communication difficulties might vary from difficulty remembering correct words to use to being mute and responding only to one's name being called, from not being able to expess oneself to not being able to understand somoen. You have to also look a lot at non-verbal communication (grunting, kicking, screaming, etc.).