Yes, you can. Leanpub supports a number of languages. When you are creating a new book, just choose your preferred language from the "Main Language Used In The Book" dropdown menu.
For some languages, your book will also have a landing page and a purchase page displayed in your book's chosen language. The languages that currently have this feature are Afrikaans, Arabic, Dutch, Finnish, French, German, Hebrew, Hungarian, Indonesian, Italian, Japanese, Norwegian Bokmål, Norwegian Nynorsk, Polish, Portuguese, Romanian, Russian, Serbian, Slovenian, Spanish, Swedish, and Turkish. If you have any corrections to suggest for our translations, or if you would like to help us add a new language, please email us at email@example.com. You can also contribute to our localization efforts on GitHub: https://github.com/leanpub/translations.