"And Goth, it's an English production. Should they have been speaking English with Dutch accents?"
I would put Anne Frank in the category of "well known" person. So actors should make every attempt to portray them as close to reality. For example, Anthony Hopkins didn't keep a British accent when he portrayed Nixon. I think for this they could have done something more than Standard British English.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.