"Turn around" could mean to face the other way, or turn all the way around to show off a dress, for example. "Turn back" is a directing someone to not just spin or face the other way, but to then go back. You wouldn't use "turn back" for the examples of showing a dress or having someone face the other way. But I'd argue you could use "turn around" any time you would use "turn back", and it would be more common (at least in the US). But "turn around" is not as close a literal translation and was not accepted.