Unfortunately the only option at the moment for this message is English. We will scope this to the book's language someday, but we can't promise when, sorry about that.

In the meantime, if you wouldn't mind providing us with a translation, we will note that for when we do get around to improving this (most translations you'll find on Leanpub were actually provided by authors in this way). You can email translations to the Leanpub team at hello@leanpub.com.