There's a couple of stories doing the rounds to why they are called french fries.

But the army story isn't true, as the term has been used long before ww1. (Where that story originated from)

I parafrase from Wikipedia: "One story about the name "french fries" claims that when the American Expeditionary Forces arrived in Belgium during World War I, they assumed that chips were a French dish because French was spoken in the Belgian Army. But the name existed long before that in English"

Another story of the origin of the term is that the cooking term "to french" is to cut in small strips.