Via the Daily Mail:
A princess called Charlotte — a name that is French in origin — is a nod to her grandfather, the Prince of Wales, while Elizabeth and Diana are an obvious mark of respect to William’s 89-year-old grandmother and his late mother respectively. Royal aides told MailOnline that the baby’s second names were self-evident and would ‘speak for themselves’. As for Charlotte, they pointed out that much had been written about it being the feminine form of Charles. Another royal source said they understood the couple simply liked the name – but admitted it was a ‘happy coincidence’ that both William’s father and mother would live on in their grand-daughter. For William, naming his daughter after Charles will be seen as a clear public demonstration of his love for the prince.