My experience in Germany was different then @Dirty-flow answer:
I lived in Germany for about 1 year and I drove a car registered in Romania, with Romanian plates and Romanian insurance and green card.
As I knew I can drive up to 6 months, I went to the local Police to ask about my legal options to drive the car, and they told me I do not have to register the car in Germany, that from the Police point of view I can legally drive it for unlimited time as long as the car is legally registered and has a green card released by another EU member state. They also told me to ask to the Tax office because there might be some taxes involved.
I have some friends in Denmark in the same situation and they have to pay around 6 - 9 Euro/month to be able to use their Romanian registered car there (this was 3-4 year ago, so laws might be different now).
I would say the best way is to ask to the Police station and the Tax office.