From what I remember.....Eggs were harvested from Kate, fertilized with Roman's sperm, and via in vitro, placed in Marlena. This was done per Stefano's orders by Dr. Rolf.
I don't think it was ever explained why Rex and Cassie could not speak any language, because even if raised in a lab, they would have heard voices. They knew how to walk, eat, etc. However, they did arrive in some sort of very small sort of satellite vehicle.
The fact is, their bio parents were American citizens, as was their surrogate. All being held prisoners, and having this done unbeknownst to them.