First, the work on wikisource is labeled BY-SA. So essentially you are not required to list the translator if republishing.
It is surprising not to have a translator listed. One method may be to try to locate the actual translation to an actual edition. On Google Books, for example, you can browse here. https://www.google.com/webhp?hl=en&sa=X&ved=0ahUKEwisyYXSwuTNAhWI4yYKHd2kCs8QPAgD#q=El+pr%C3%ADncipe+y+el+mendigo+mark+twain+book&hl=en&tbm=bks
Another possibility is to search on WorldCat for a physical copy of the book. Even if you don't try to obtain the book, WorldCat might give you metadata about the translator's name.
Some books simply aren't published with the translator's name. (I seem to recall that 1001 Arabian Nights has a public domain version without a translator's name).
For the record, Project Gutenberg doesn't list spanish translations of the book: https://www.gutenberg.org/browse/authors/t#a53