Langer's side face England at Old Trafford this week with the Ashes tied at 1-1 in the five-match series.
England's remarkable win at Headingley in the third Test has breathed new life into the contest and could force Australia to make changes.
They seem certain to make at least one switch, with star batsman Steve Smith available again after suffering concussion following a bouncer from England's Jofra Archer in the second game at Lord's.
Pacemen Mitchell Starc and Peter Siddle will also hope to be involved.
Langer was giving few clues away as to Australia's likely selection at Monday's press conference and said his players are still in the dark as well.
"The players don't know yet. We're going to tell them before dinner tonight and then it will probably be public after that," Langer said.
"We're very clear on the 12, not so sure on the 11. Sorry I can't give you more but it's probably better to tell the players before we tell you.
"The 12th man will be one of the bowlers. We're very clear what the batting is."
Former Australia captain Smith has scored a combined total of 378 runs in three innings in the series so far, at a remarkable average of 126.
Smith was back on the field in last week's tour match against Derbyshire, scoring 23 in his only visit to the crease, and Langer hopes he can now pick up where he left off in the Test series.
"I've just thrown to him for about half an hour and he's moving really well, so we're all hopeful that he gets back into the zone, or hits the ball in the middle of the bat," Langer said.
"He's shown that throughout the summer. He's been batting well, so let's hope he can continue that. It's important for our team and it's just nice knowing that he's back in the team."