When it comes to crediting people, it's best to let themselves decide, unless there are some rules/guidelines that enforce a specific scheme that you must follow. So, if you still have contact with Alice, just ask her.
Perhaps she just doesn't want her legal name out there, but wouldn't mind being credited by a pen name. This is by far the easiest route to give proper credit while technically maintaining anonymity -- though that's under the assumption that the pen name Alice gives you wasn't used by her elsewhere and can't be linked to her. So, if you go with this, you could credit "Bob, Ooker and Carol", with Carol being Alice's alias.
You could just credit her as "Anonymous" or a variation thereof, if that is what she prefers. Yes, "an anonymous translator" is a mouthful, and unnecessary. There is also absolutely nothing wrong with saying "others" when it involves just one person.
For some projects, people might not want any reference to them whatsoever, and there are many possible reasons for that. For example: people might be able to deduce who she is, even if you credit her as Anonymous or a pen name, just because someone knows Bob and/or Ooker closely, and could guess who "Carol" or "Anonymous" are through that, rendering the purported anonymity worthless. If those are her concerns, and she wants to not be credited at all, there's no reason not to respect that -- just pretend she does not exist for the sake of this argument, and simply credit "Bob and Ooker".