Just to add, my niece would refuse to speak to family/friends etc who said hello, goodbye.
The whole family thought of it as rude tbh.
Fast forward a few years to a holiday in Disneyland, Paris, and my niece got lost. She was approached by a member of staff, who repeatedly asked her name, what was her mummy and daddy's name, to which she would not reply. She remained silent.
The member of staff then assumed that perhaps my niece didn't speak English, and he proceeded to address her in French, German and Spanish. Still no response.
20 minutes later, by pure luck, my sis who had been frantically searching for her spotted her with the member of staff, and called over to her, at which point the member of staff realised that my niece was English.
Member of staff explained that if your child refuses or cannot speak to another person, they should always always carry ID around with them, with some form of contacting the parents, no matter what age they are, since it reunites parents with children much faster.
Since this day, my sis, who had previously made excuses for niece's behaviour, saying she was tired or shy, or not feeling well etc, spoke to niece, and explained that she must speak to people if she needs help and to always be polite. My niece is ok with it now.