This is always somewhat subjective but this is our take:

On the Dutch side:
Many times tip is included as a line item on the bill, if that's the case that's all we do unless the service is exceptional, then we may put another 5% on.
If no tip is included we usually do 15%, unless the service is exceptional, then we may put another 5% on.
If it's a happy hour that really reduces prices then we leave 20 to 25%

On the French side supposedly employees are paid more so we usually leave between 10% and 15% depending on service.