When most Leanpub authors want to publish a translation of one of their books, they create a new book themselves. This means they are the primary author of the new book (when you create a book yourself, you are automatically the primary author).
Next, they add the translator as a co-author. That way, the book will appear on both the primary author's Leanpub profile page, and on the translator's Leanpub profile page.
After you create a new book, you can indicate it is a translation of another Leanpub book by going to the "About" page for the new book here:
...making sure to replace YOUR_BOOK with your book's unique web address.
You can also access the "About" page from your book's "Overview" page here:
On the "About" page for the new book, scroll down until you see the "TRANSLATED FROM (OPTIONAL)" dropdown:
Next, click on the dropdown, and select the relevant book:
Please note that this list will include all of your books, including unpublished books!
After you have selected the relevant book, click the blue "Update Book" button at the bottom of the page:
On your new book's Leanpub landing page, in the "About the Book" section, it will now display a sentence about the translation, with a link to the translated book: