There are (A) credit card only machines and (B) machines that take either cash (ie, notes/coins) or credit card. All machines have an English button /often other languages too and you can easily select the train, seat etc if relevant, and pay.
Technically it is considered theft of services to board without a valid ticket. So if the conductor really wanted to cause you trouble, he/she could have you arrested. Most however would probably never go to those lengths, unless you gave them a reason.