Blackthorne was an amazing Linguist!
If the story is to be believed, Blackthorne had an amazing nack with languages. Obviously English was his native tongue. He must have spoken excellent Dutch to be able to communicate with the Sailors on the Erasmus. When he got to Japan he was fluent in Portuguese as was shown by his conversations with the Jesuits and Mariko. Also he was well on his way to speaking the Japanese language at the end. Amazing for a 17th century Sailor!