Thank you for your interest in translating a Leanpub book!
Leanpub is a self-publishing platform. Authors on Leanpub own the copyright to their work, and any translation arrangement must be negotiated with them.
So, if you would like to translate a Leanpub book, what you should do is contact the author of the book, and get their permission to translate their book, and work out some kind of revenue-sharing arrangement.
You can contact the author by clicking the "Email the Author(s)" link on the landing page for their book, unless they have disabled that feature. In that case, you can reach out on social media or make contact some other way. Authors often have blogs where they have contact forms or even display their email address.
Authors typically love to hear it when someone is interested in translating their work!
Setting Up a Translated Book
When you are publishing a translation of a Leanpub book on Leanpub, typically the original author creates a new book on Leanpub, indicates that the new book is a translation, and then adds the translator as a co-author, setting up the royalty split as agreed.
For information on how and when Leanpub pays royalties, please see this article.