In France, in absence of contrary indications (ie: absence of signs, traffic signals, or line markings), it's "Priority to the Right".

So if it's an actual roundabout (aka, "rond-point"), then normal traffic rules apply for intersections: Priority to the Right. Vehicles already on the roundabout must yield to cars entering it.

Often, you have what is referred as "Carrefours à sens giratoire", which can very much look like "rond-points", but priority is to the vehicles already on the roundabout. For this reason, there will be a yield sign at the entrance of the roundabout to make it clear there's a special rule that applies to it. Sometimes you have traffic lights as well.