Do you have any examples of American expressions young generations are using in French?
This is caused by people active in English-speaking communities translating memes literally and spreading them in their native language communities as-is.
As the meme spreads, monolingual speakers begin using the same format and eventually they reference it off-line.
Having said that, SOTA models got much better at this kinda stuff. They're quite able to write in a way that is indistinguishable from a native speaker, colloquialisms and all, with the right prompting.
But SOTA models are also expensive. Most automated translations are done with something way cheaper and worse.
You're right that memes are getting translated literally and spreading, but in terms of pure volume, LLMs doing the translation dwarf humans doing the translation.
Building a bot that picks whatever posts are trending on Reddit/imgur/etc, automatically translates them, and then posts them in target languages on social networks is an easy way to accumulate likes+followers; then those high reach accounts get used to push whatever makes money.