I've been doing some work on Wikipedia's History of Games articles (* cough*), and I would suggest that you be extremely cautious in accepting some of that information without at least a grain of salt, particularly in reference to card games and games of chance. However, that would lead me to wonder why they used the term Jack in that game instead of Knave. Wiki suggests the term comes from a game called All Fours. How about other languages besides English, French, and Spanish? Let's try to get to the bottom of this people! They probably didn't want another R, so what does the D stand for? I have a deck of cards here from the Casino de Montreal and here is what the French decks do:

However, what letter do they represent the two ranks with on Spanish cards? It could get confusing if they were both an R. A bit off topic, but the word for King and Queen in Spanish are Rey and Reina, respectively. In Spanish it is called a Sota, which I believe means helper. Perhaps it would help to shed light on this mystery to know what they call jacks in other languages. According to Wiki it is 'a male servant of royalty.' Perhaps there is a connection with the term jack as used in 'jack of all trades.'

In 1864 an English card maker used a J instead of Kn, to avoid confusion, with the J standing for Jack. Wikipedia tells us the term used to be Knave, and was designated with a Kn. I've been playing card games for about 40 years about I don't think I ever stopped to ponder what is a Jack until recently.

French card game popular in casinos