OM (Olympique de Marseille) and PSG (Paris Saint Germain) are rivals in football, and for your own safety, do not go to Marseille with a PSG shirt beacause they can take this as a provocation. (This is not a joke).
Just don't talk about Paris in Marseille. Marseille is beautiful enough for you to forget about Paris :)
Most house parties in France expect people who are invited to bring drinks or snacks to share with every one. Despite what you could see in other countries in Europe where what you bring is what you drink, in France what you bring is no more your as soon as you enter the house party lol
If really you forgot to buy something, just ask as soon as you arrive where is the nearest "épicerie" to buy a little something for the guest and the party.
No telling people to enjoy their meal before eating can be really really seen as having bad manners.
If you can't pronounce the words, a simple : have a nice meal, will be okay, i guess haha.
Waiters can really be upset if they don't get a "Bonjour" (good morning / hello), so if you want to be well served better not forget this.
If you forgot, just excuse yourself as soon as you remember.
Though it may seem obvious if you're from europe, it's not obvious for everyone. Anyway it's not respectful to point someone with your finger
Describe the person instead