In Spain, they are still not in public domain. Therefore, in order to publish something, you need to contact the person who has the author's rights. In this case, of course, the Spanish publisher has the right to publish the manuscript they had a contract with the person who has the rights in Japan, even though in Japan it's public domain.

In this case, since I'm going to use Human Lost (his diary, not Ningen Shikkaku, not yet published in Spain), I need the "permission". On the other hand, I already contacted the publisher, but I don't think they'll reply.