According to our friend CMOS, 16th edition, section 8.60: “Personal, national, or geographical names, and words derived from such names, are often lowercased when used with a nonliteral meaning.”
Basically, this means that even though you think certain words should be capitalized, when they are being used in a nonliteral sense, they are lowercased.
Here’s an example:
At first it seems like a word that should be capitalized: Swiss cheese. After the Swiss, of course. But this is not literally referring to cheese that is found in Switzerland or eaten by the Swiss. It’s just a type of cheese that’s, to be more specific, “a cheese that resembles Swiss emmentaler (which derives its name from the Emme River valley)” (CMOS 8.60).
CMOS offers a list of similar words, some of which are shown below:
So yes, french fries are lowercased unless you are speaking literally about fries found in France. This isn’t a grammatical quandary you will run into all the time unless, perhaps, you frequent McDonald’s and then like to write emails to your friends about the delicious french fries you just ate.