Jehochanan (Hebrew)
-->
Johannes (Greek)
--> Johann/Johan (
German / Low
German)
-->
Jan (Flemish / Frisian / Low
German)
--> Janken/Jankin ("little
Jan", Flemish / Low
German)
--> Jackin (Flemish)
-->
Jack (Flemish / English)
Another short form of
Johann is Jon/John - so there you've got the connection.
By the way - all the J's are pronounced like Y, even
Jack and
John, except in English.
Jan and
John can be short for
Jonathan as well.
Another way "
Jack" came to the English language was via French "
Jacques" (French form of
Jakob).
Regards,
Satu