You CAN register someone else’s name since there is no technical blocker for you to do that. However, it may cause you legal trouble. As in any other offline or online situation, the name may create a conflict with the name of a person and such person may have a claim against you to cease using that name as ENS domain and if you don’t follow such a claim, a German court (if competent) may issue a judgment against you. The problem as of now (March 2023), is rather that there is no effective way of enforcing any such judgment due to the anonymous and decentralized nature of the blockchain. But make no mistake: this does not make the action legal.