I think that initially the onus would be on Youtube to figure this out. They have some very intelligent engineers. For example, if the Youtube client is receiving affiliate funds then they are easy to ID and fine. If they are random people then Youtube would have to share the violation data with the other countries and the US or UK would have to pressure those countries to participate in fining the end user. There could be financial incentives for the foreign country to participate. They can also just force label a video to be adult as they do today when enough people report it which is admittedly not uniformly applied.