*This achievement is missable*
To get this achievement, you'll need to rent another book in the Le Beaux Kuckelle library. The book is called Learn to Speak Japanese, however you can't get this book right away. In order for this book to become available, you'll need to reach the part where Wortten challenges you to a cook off. Speak with the Japanese judge, and you can then either go back to learn Japanese and get the achievement, or you can do it after the cook-off with Wortten. Either way, after you talked with the judge, go back to the library and the book will be there. Rent the book and a cutscene will come up showing how DeathSpank learned Japanese.