You make diminutives in
German by adding -chen, right? So
Margarete becomes
Gretchen.
Languages change their ways of doing things, but way back in the middle ages, English, Dutch and
German all did much the same thing - Dutch and English went for the K sound rather than the CH sound of
German, which is why we are still familiar with catkins (on willow trees) and with dear little lambkins! We don't make new words that way any more, but some have lasted.
One of those is
Jack. You can see the progression:
John = Johnkin = Jokkin =
Jock if you're Scottish,
Jack if you're English. (Over the centuries, it's often the consonants that stay the same and the vowels that change a bit.)