If it's too long people might use In Scottish and in English: Colin In Italian: Nico In German and in Dutch: Klaas In German: Claus In German: Klaus In Finnish: Launo In English: Nic In English: Nick
For the opposite sex use: In Romanian: Nicoleta (F) In Greek: Nikoleta (F) In English: Nicola (F)
See also In Italian: Nicoletta (F) In French and in English: Nicole (F) In French: Nicolette (F) In Esperanto: Nichjo (M) In English: Nicky (B)