My company also states that English is the designated language on board for communication between crew members. ( I believe it is a licencing authority requirement) This even means "chats" between crew members who may both speak a different first language. I should imagine it is more to do with maintaining good CRM among the rest of the crew rather than some crew members talking in a different language that nobody else on the crew can understand. It is a bit like whispering. It can be seen as rude and antisocial, even if it isn't meant to be.