What about "rendezvous", or "etiquette", or "RSVP", cliche, nuance, etc? Do you consider those French or English?

As you say, the line is very very blurry.

My favorite in the vicinity of etiquette and rendezvous is the "double entendre", very French sounding, but not French at all. That and something being not a person's "forte" which when correctly pronounced is just fort, but through confabulation with a musical term from Italian; forte: to play loudly, sounds more French to English speakers when mispronounced. C'est la vie.

Japanese loanwords really tickle my humour; バイト "Baito" : a casual, part-time, non-serious job. From the German "Arbeit" which is serious, macro-level employment or exertion.

Rendezvous and cliche yes. Nuance, etiquette, RSVP no. It's instinctive so I can't explain but maybe because rendezvous and cliche require using French pronounciation. On this I think you could find more differing opinions than there are possible answers.