Klaus is Japanese Because it's Written as 倉臼 - S1.E36

Klaus is feeling depressed after Nagi gives him permission to retire after he tells her about a dream he had, where he retired on a coffee plantation in Brazil. After Maria tries to convince Klaus to stay, Klaus decides to challenge Hayate to see who is the better butler. Klaus fails at this and has his mind set on retiring. Maria comes up with a plan that involves Hayate's female alter ego, Hermione. This plan fails as well and just before Klaus is about to leave, Nagi thinks of a way to make Klaus stay. Her plan succeeds and Klaus decides to stay as a Sanzen'in butler.