Belgium is separated into three very different regions: Flanders, Wallonia and Brussels. There is a very big disparity in terms of wealth and demographics between those regions as well as some significant regional tensions. Major political conflicts often appear and I would definitely avoid speaking about it with Belgians, unless one is very acquainted with his interlocutor.
For the same reason, I would additionally suggest learning a few words of French and Flemish in order to display some cultural sensitivity, However English seems to be the most non-offensive language to all demographics. This will help in not discriminating and remaining politically correct. Most people in Brussels and Flanders have a good level of English although it is not so widespread in Wallonia.
In order to keep their unity despite their disparities, Belgians are used to compromises and I would avoid direct confrontations. Though, there are big differences between French and Flemish in approaching people, Flemish are more direct while Walloons are more diplomatic.
Finally, I would pay special attention to the social system; people have many constitutional rights and are well aware of all of them. Be it in consumer rights, employment laws or health care, it is important to play by the rules.
I will restrict the contents of my essay to Paris, some disparity can be seen throughout the different regions, especially the South.
Dealing with Parisians is notoriously challenging. People form the southern regions in France are much more amiable and laid-back. The French are very formal and one should only call a person by his first name in a non-work environment, and only after being invited to do so. In a professional situation, I would avoid it all together, except eventually with a select number of colleagues, where the polite form for the second person (“vous”) should be kept at all...