Is using foul language and calling people out of their name part of dementia behavior or part of past life experiences?

Sadly this is common in residential settings for elders. In a nursing home, it is usually ascribed to dementia and not in the person's control. If that is true, the insults are distribute randomly. Took care of a minister once - his family were appalled at the words coming from. his mouth. We reassured them we were fine with it, he scattered those words all around him when they arose.
If the elder only curses at the care staff, or the non Caucasian staff, or fixates on one particular staff person...then the elder likely does have some control. (based on years of work in geriatric and home care settings.)
I was caring for a lovely lady in a lovely home, and her 2x/week HHA went on vacation for 2 weeks. The substitute aide was not the same skin tone as the assigned HHA. She endured 2 weeks of verbal harassment and occasional pinches. She didn't report it to me until afterwards, she knew nothing would be done as it was only 2 weeks.
It can be effective to set a boundary with the care recipient - even with some dementia, many elders can learn important limits. Sometimes not. It is worth trying.

Yes, they lose their filters. Anything that comes into their mind comes out of their mouths.

I was at Moms NH visiting. A woman was calling her aide every bad name in the book calling her a whore, using the F word. I walked by and the woman said "what a pretty blue top u have on" I said "thank youcp" she said "your welcome" and then went back to berating the aide.

Really, if they were able to realize what they are saying, they would be so embarrassed.

This video may be helpful.

It features Teepa Snow, and can be found on YouTube,

It’s called “Common Issues in Dementia” Teepa addresses the issue you’re asking about, along with many others.

https://youtu.be/u5QMeQpkPhA

It could be either or some of both. Each dementia patient is unique, though there are common behaviors, not everyone has like experience

In dementia the brain has many disconnected parts. When the frontal lobes are damaged, the patient loses the ability to filter and censor his thoughts and urges. All his primitive instinctual impulses (sexual, aggressive, etc.) become lose. This behavior is more frequently observed in fronto-temporal and vascular dementias. Of course, all past life experiences are archived in the brain and they may color the dementia's symptoms.