UK Copyright on Tolkien runs to 2nd September 2043. It is Tolkien’s death plus 80 years, unless the law is changed.
US Copyright is publication date plus 95 years. So “Aotrou and Itroun” will possibly be in public domain in the US in 2040, unless the law is changed. My understanding is that books originally published outside the US by non-Americans are exempt from this requirement, if they are still under copyright in their home country.
Unpublished material currently becomes copyright when it is first legally published with the permission of the current owner. For a letter, the current owner is the writer of the letter or the legal heir to the writer of the letter, which may be a different person from the current owner of the physical or digital letter.
Discussions on changes in the law tend to attempt to reconcile copyright agreements in various countries.
Presuming that “Aotrou and Itroun” is still copyright by
The Welsh Review which was last published in 1948, then “Aotrou and Itroun” is owned by the current copyright owner of
The Welsh Review material. However
The Welsh Review may have only purchased first rights to the poem, this being a norm for periodical publications, in which case “Aotrou and Itroun” is presumably owned by the Tolkien Estate along with most of J. R. R. Tolkien’s other works.
See
http://tolkiengateway.net/wiki/Pesma_o_Otruu_i_Itrun for information on a bilingual Serbian/English edition published in five hundred copies in 2002.
Since the HOME books have sold far better than originally guessed, if the complete
Fall of Arthur sells well, then we may see some of the other missing works