The correct thing to do is to contact the authors directly: translations on Leanpub are arranged by the authors themselves, typically for a revenue % of the translated book. Most authors have an email the author form at https://leanpub.com/THEBOOK/email_author/new

Leanpub handles the royalty splitting on the translated book.

Note that you need to be able to receive PayPal payments in order to be paid royalties.