Squanto’s people had already had contact with Europeans. Squanto had already been to Europe. That’s how it is that he knew English.
“Squanto had already been to Europe. That’s how it is that he knew English.”
Yes, he was captured because his entire tribe mysteriously died prior to contact with Europeans. His first contact was to be enslaved.
As did, in fact, the tribe native to the area where the pilgrims settled.