First of all, don’t sweat the non-French talking : Paris gets heaps upon heaps of tourists, and most Parisians speak a passable Angleech. You’ll be fine.
Seconded. It’s something that comes naturally to us, to the point that we don’t even notice people’s bonjours/au revoirs, but when it’s missing it’s… well, not really shocking or offensive but… well, it’s just Not Done. Tthe more assholish among us will even refuse to talk to you at all until you’ve said it. Doesn’t matter who you’re talking to, whether it’s an employer, employee, the cashier at the grocery store or the random guy you want to ask directions to : open with “bonjour” and close with “au revoir”.
If you’re a woman, your co-workers might subject you to “la bise”, a slight peck on each cheek. It’s ok, the guy (or gal) is not hitting on you ! “La bise” is to women what handshakes are to men. Either get used to it, or premptively counter it by offering your hand to shake. We’re not big on hugs, even among close acquaintances.
Paris is a very rushed and “frowny” city. Every NYC stereotype probably applies here too : people don’t smile all that often, and people are always in a hurry. Bar waiters doubly so. Don’t be surprised (or offended) if you get massive eyerolls should you take more than half a second to order what you want in a café.
Speaking of cafés, there’s a little something to know about if you’re a coffee drinker : if you drink your coffee black, then all is fine, just order “un café”. If you want milk in it, do not order “un café au lait” however, because they’ll bring you a whole bowl of milk with some coffee in it. The proper way to order coffee with a drop of milk in it is “une noisette”.
For beer, you’ll want to order either “une pinte” (half a liter, or 15ish ounces) or “un demi” (half a pint, quarter of a liter, ~8oz). Careful about ordering just “une bière”, some unscrupulous cafés will try to gouge you by bringing you gigantic, overpriced glasses or even pitchers.
Same about taxis : they can smell the tourist in you, and they’ll rip you right off if they think they can get away with it. We’ve got a great & cheap subway system - learn to use it ASAP. Buy yourself a “carte orange” - the monthly pass.
Money is something of a taboo subject - I know Americans have no hang ups about talking about how much they earn, how much they’ve got, their salaries etc… we don’t. Over here, it’s Private Business and uncouth to talk about. Same thing about religion : keep it to yourself and don’t expect people to tell you about theirs. We’ve had enough religious wars over the centuries to learn to keep that stuff close to our chests :D.
That’s all that’s coming to me right now, but I do recommend reading “A Year In the Merde” by Stephen Clarke. Some of it is exaggerated, but it’s spot on for the most part.