Learn English – the meaning and etymology of “cod-French” accent

Straight Dope has a good explanation and here is the answer from the article:

For also in the 1840s, pomme frites ("fried potatoes") first appeared in Paris. Sadly, we don't know the name of the ingenious chef who first sliced the potato into long slender pieces and fried them. But they were immediately popular, and were sold on the streets of Paris by push-cart vendors.

Frites spread to America where they were called French fried potatoes. You asked how they got their name--pretty obvious, I'd say: they came from France, and they were fried potatoes, so they were called "French fried potatoes." The name was shortened to "french fries" in the 1930s.

By the way, the verb "to french" in cooking has come to mean to cut in long, slender strips, and some people insist that "french fries" come from that term. However, the French fried potato was known since the middle 1800s, while the OED cites the first use of the verb "to french" around 1895, so it appears pretty convincing that "french fried potatoes" came before the verb "frenching." The origin of the name is thus the country of origin French and not the cooking term french.

Here is a passage about how it is introduced to America from a book called "Potato: A History of the Propitious Esculent" By John Reader:

French kiss is from 1918 according to The Routledge Dictionary of Modern American Slang and Unconventional English.

The following source cites its first usage in the WWI Private Lindner's Letters:

• While "French kissing" began appearing in conversation during the 19th century, it took off in the early 20th century when it first appeared in the World War I book Private Lindner's Letters: Censored and Uncensored of 1918. "I have decided to become a linguist," he wrote. "Being able to read French fluently and speak it wretchedly, and to speak German connectively but not to read it at all, I am taking up Luxembourg, which is a wonderful blend of the two, a sort of liaison between tongues. (Not to be confused with French kissing.)"

• In fact, if we're going to credit anyone, then it probably should be the British and American soldiers of World War I. Upon returning home from continental Europe, they showed their wives and girlfriends what they learned from the amorous country and called it a French kiss.

(www.popsugar.com)

As noted in Quora, the term French was used in a number of prurient expressions at the beginning of the 20th century:

• I believe the term originated in England at a time when adding the term "French" to something already salacious made it even more prurient. For example, syphilis was called the "French pox" by Englishmen and condoms were referred to as "French letters," for some reason. The "French kiss" is simply taking a relatively chaste kiss a step further to a deep, tongue-involved kiss and attaching further sensuality to it via the term "French." Of course, that sort of kiss has probably been around as long as there have been human beings, and certainly isn't the invention of the French.