We have no special requirements for translators. From our perspective, the only thing that matters is that you have the rights to publish the translation, and that you indicate somewhere that the book is a translation.
We have no formal process for publishing books that are translations; a translation is a book just like any other book.
Regarding royalties, what you do with the royalties you earn from the book is up to you, as long as you abide by whatever deal you have made with the original author, or the copyright owner, which has nothing to do with Leanpub.
If you want to translate a book that is already published on Leanpub, we do have some features that you may want to use. Here is some guidance for people who want to translate Leanpub books: http://help.leanpub.com/en/articles/117487-i-would-like-to-translate-a-leanpub-book-what-should-i-do.