The type of fry has no bearing on whether or not a fork needs to be used. We have found it depends more on where you are eating. If you are at the Burger King on base, then you will blend right in with all the other Americans eating their fries with their fingers. If you are at a German eatery, use a fork (even at the German lunch stand on base)! The only place outside of base where we have seen fingers being used is at the McDonald's in Trier. They even have french fry stands, and they will offer you a small fork for you to eat them!
See example below of the little french fry forks. I will have to find out where I can buy these!!
And for my Mom, please notice the Coke that I am drinking, not beer or wine!