"Sophie Ecclestone has finished school now and has had a great summer domestically. Her selection will provide another option to our attack," said coach Mark Robinson.
"It's a hugely exciting tour and one we are really looking forward to. Australia on their home soil will be a further test of the progress we have made.
"We said before the World Cup that the final at Lord's was never going to be our destination, it was just part of the journey, and that journey continues with this tour."
Lancashire's Ecclestone has played two ODIs and two T20s but has not featured for England since the tour of the West Indies in October 2016.
Langston, 25, was an unused member of the squad for the World Cup, which England won with a dramatic final victory over India at Lord's in July.
As well as the touring party, England will have pace bowler Kate Cross and wicketkeeper Amy Jones in Australia, with both playing domestic cricket for Western Fury in Perth.
And it will be the first time Knight has led her side into an Ashes series.