Ben at Toys and Techniques mentions the Welsh writer Owain Owain. It appears his books have never been translated into English (and probably never will be). I have the sense they’re not in print in Wales either.

From wikipedia:

His science fiction book entitled Y Dydd Olaf (“The Last Day”) was described by the Welsh literary critic Pennar Davies in the book’s preface: “Nothing like this book has been seen before either in our language or in any other language. We should rejoice that such brilliance exists in Welsh writing.“

this photo via Gwenno