The question why Jung and his colleagues wrote "Man and his Symbols" in English is pretty much explained in the introduction of the book by John Freeman, who oversaw the production of the book.
The idea for the book came after a documentary on Jung was broadcast by the BBC and proved to be relatively popular. After its success Freeman approached Jung to write a book about his theory for popular consumption (as opposed to his usual rather difficult writings), and after some back and forth Jung agreed.
Since the book was always intended for publication in the UK, it's not surprising that it was written in English. Jung and his colleagues (like most German speaking intellectuals) were fluent in English, so writing the book in English posed to be no problem for them.