What's the best gift for your French friend's birthday?

Look at what his favorites are and what you can afford. A nice bottle of wine?

But it doesn't need to be too expensive, because you don't know each other very well, and if you buy it expensive, I guess they will have a psychological burden.

Or maybe a bottle of wine, a small potted flower (if your coworker likes flowers) and a nice box of chocolates, all in one of those gift baskets that look nice and big. The price should be 100€. 50€ is enough to buy a bottle of wine (unless he specializes in wine, then there's no need to give wine.) Not bad for an average relationship.

Or you can see if he has a habit of collecting anything, and give him something he collects and opens that he probably doesn't have...

Haha but chocolate is rarely something the French don't love.