In the anime, Nicolas cannot speak properly due to his disability. He has to use sign language in order to communicate with others. Is the sign-language used in the anime authentic? Which sign language is used if it is?
The sign language used in the anime is the Japanese Sign Language. For example:
Means "thank you (ありがとう) in JSL. The ITEC website, which teaches JSL to English speakers, details how to make this sign:
The left hand is held palm-down in front of the chest, fingers pointing to the right with the open right hand's blade edge touching the back of the left hand, fingers pointing toward the interlocutor. The left hand's position is held, while the right hand is lifted to chin height.
One thing to keep in mind is that the setting implies that the characters are all speaking English or some kind of European language. The language used in the newspaper and currency for example, is English. The sign language used in the anime is authentic, but it may not be consistent with the setting of the anime itself.