What you should do is contact the author(s) of the book, and get their permission and work out some revenue sharing arrangement. Authors on Leanpub own the copyright to their work, and any translation arrangement must be negotiated with them.
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.
A translator should be set up on Leanpub as a co-author of the translated book. We can automatically split royalties between co-authors, however the primary author intends.
In addition to being a co-author, on Leanpub there is also a different concept, called a "contributor". A contributor is not a co-author. Instead, they are someone who just adds a bit of content or makes some kind of other contribution (say, editing). A contributor is not eligible for royalty payments or royalty splitting. Instead, if an author wants to pay them, this has to be done outside of Leanpub.
So, if you want to translate a Leanpub book, you should ask to be a co-author of it, not a contributor.