The Krankenkasse would be a great place to start (they pretty much all have some English speaking help if you don’t speak German) as they would coordinate the Pflege that would be covered through the Pflegeversicherung.
This organisation in Frankfurt (HILDA) helps people with dementia and their relatives. They offer group help and house visits www.buergerinstitut.de/demenz/
I think it would be the Amt für Soziale Sicherung or the sozialamt depending on which state you're in. Germany varies enormously by state. If you knew which Krankenkasse she's with they could also advise.
My elderly relative lives in Frankfurt on her own and we are concerned she may have dementia. We have no way of contacting any of her friends or Doctor as we do not have the information. Is there a social services or similar service that we could contact to call on her direct? Thanks in advance