I have a very strong suspicion that J.T. DeBacker was from Belgium. Especially with the spelling "DeBacker", if it were Dutch the spelling would have more likely been "Bakker". Probably a very common name since it means "The Baker", the name Vandenheuvel literally means "From The Hill" also a very common name.

Since they left from Antwerp, also lends to the idea they may have come from Belgium. There are no absolutes here, but my guess is they were dutch speaking (flemmish) Belgiums.