If our experience at qarrtsiluni is any indication, getting permission from living authors tends to be relatively easy, as Ren says (presuming one knows the language well enough to draft a letter in it). It’s the not-so-famous, out-of-print authors who have been dead for several decades who are a bit harder. Alex Cigale also assures me that it’s acceptable practice to go ahead and publish translations for which one has failed to track down the copyright holder, accompanied by a note stating that a reasonable effort had been made. I’d still feel nervous doing that if I were a print publisher, though.