A dialect called "Jersey Dutch" was spoken in Northern New Jersey until the 1930's. ...
The Dutch Reformed Church congregation in Demotte Indiana did not cease church services in Dutch until the very early 1960s. When I attended the town festival circa 1999 - 2004 the elderly folks were still showing off speaking in their version of Dutch
NE Illinois around St Anne had residual French in use into the early 1960s. While the Amish & German Baptists in my region of Indiana use little German vocabulary their grammar is as much German as English.