Hmm, my parents and relatives speak more than one language. It may delay dementia, but I believe exercise, and social are required components that will prevent dementia. For instance, my maternal grandmother used to speak German, Low East Prussian and American English. She was stubborn with some issues and PTSD. She wasn't interested in socialize with others except families and relatives. We always tried to talk to her, but she just chose to not engage in conversation with us, and it was driving me, and her grandchildren crazy. That's when her dementia started and she couldn't take care of herself longer. She passed away the day before her birthday.
My Mexican father is in his 90s now. He engages in conversation with friends, relatives and family. He loves to go out all the time and meet people. He is very chatty and charming. He speaks Spanish and English. He takes fish oil daily, which helps his memory. He still drives a car. If you meet my father, you will never know he is in his 90s. He loves to gamble, and read all the time! He can take care of himself. However, he has to deal with two cancers for 5 years and he keeps it under control. His medications are not very good for his memory. That's one of the reasons he takes fish oil.
Also, Barbaro has some good points to consider regarding delaying and preventing dementia. Thank you both for sharing. This is a topic I am always interested in as I am in my 60's now, and I am always looking for ways to keep my health intact and my mind sharp.