Tried to be fancy

Once I had adapted à la mode of using fancy words with a hope of improving my English. I tried finding several fancy words from different sources and presented a writing below with an intent to impress my English teacher. These are of course quite complicated and I also do not remember/know what most of these word mean. However, my English teacher, with compassion, took some time to read this and gave me long, elaborated comments.

Words that are italicised are my writing and the rest are my English teacher's comments.

So, as you've seen, English has borrowed certain terms from French and Latin. In using these in ordinary writing, and especially conversation, though, we run the risk of appearing pretentious to others, "showing off" our vocabulary in what might look like we're trying to appear better than others. So we have to be really careful. Sometimes we can use such terms in a joking way, but this takes real skill and cultural awareness to do right.

Anyway, here's the first part with my comments interspersed:

"A few days ago, while going for a dinner (we usually say: going out to dinner) with some of my friends, I happened to see a bon vivant nonagenarian (bon vivant is a noun, and while nonagenarian is too, it's somehow more "adjective-like" because it describes the person's age, and so should go first: nonagenarian bon vivant; now, when you choose a French term like "bon vivant", I imagine that the old man is either European, or has something European about his style) enjoying dinner with his convives (this is a word that's so rarely used, I had to look it up to see if it *was* a word; I don't think I've ever used it or seen it used my whole life) jovially in a convivial (just as a style note, unless you're making a kind of ironic joke, using too similar words like 'convives' and 'convivially' in the same sentence is redundant) atmosphere. He was trying to behave with a savoir fair (this is spelled savoir faire and never takes an article in front of it) and had a certain je ne sais quoi (I assume you know that this translates as "I don't know what" and means a quality that's hard to put into words; it has a positive tone, but again, a person who uses it is usually regarded as a little prideful or pretentious) that charmed everybody over there (unless the person is physically very far away, you can just say 'there' instead of 'over there'."

"Seeing this made me feel that the enjoy (this is a verb; enjoyment is the noun and it doesn't need an article in front of it) continues ad infinitum (This literally means "into infinity" and is used to say "forever" but in a slightly more negative tone than you're using here. It's often used to express boredom about something that *seems* like it's going on forever: "He talked about his vacation and every little bit of the food they ate ad infinitum." The way you use it here is a little too much of an exaggeration because people are, of course, finite.) irrespective (good use of this word) of age."

"Later he disappeared in the cul-de-sac while I tried following him."

This is OK, but might be best like this: Later he disappeared into the cul-de-sac when I tried following him.

"While" gives the sense of an action unfolding over a period of time; "when" gives the sense of an action "disappearing" happening right in a particular moment.