The 24-year-old Langerak was on the bench for Dortmund's 4-1 win over Real Madrid in last week's Champions League semi-final, first leg, but played the 90 minutes in the win at Dusseldorf.
After Dortmund took a 2-0 lead, Dusseldorf midfielder Adam Bodzek pulled a goal back with a header two minutes from time before Langerak pushed defender Martin Latka's shot onto the post to give his side three points.
"I am really happy with the game, Mitch pulled off a series of saves," said Klopp, as Langerak made his third Bundesliga appearance of the season as understudy to captain Roman Weidenfeller.
"If we had conceded a goal at the end of the game, it wouldn't have been undeserved."
Langerak joined the club from Melbourne Victory three years ago.
Midfielder Nuri Sahin put Dortmund ahead with a stunning goal from 35 metres out before Poland captain Jakub Blaszczykowski scored in the 70th minute, 60 seconds after coming off the bench.
Dortmund travels to the Bernabeu on Tuesday evening (Wednesday morning AEST) for the second leg with the Ruhr Valley club bidding for its second European final appearance having won the 1997 title.
Its director of sport Hans-Joachim Watzke said his side's win over Fortuna proved that there is not just one side in the Bundesliga with great strength in depth.
Dortmund coach Jurgen Klopp made 10 changes to the side which had beaten Real Madrid 4-1 in the Champions League last Wednesday, keeping only defender Mats Hummels in his starting XI.
They were nevertheless too strong for Dusseldorf and cemented second place in the table, which it can clinch for certain next weekend.
"It has been said for weeks that there is only one club who can rotate and still win games," said Watzke.
"I'm very pleased that we have now done that successfully for a second time, just to show people that we also have players who can prove themselves in such a Bundesliga match.
"I am also very pleased that we got this result so that nobody can criticise us of playing only half-heartedly."
Nuri Sahin was one of the beneficiaries of the rotation, staking a claim for a place against his former club on Wednesday (AEST) with a dream goal to set Dortmund on its way to victory.
He also set up the second for Blaszczykowski and praised the atmosphere within the club, in spite of the ongoing transfer speculation surrounding the likes of Hummels and Robert Lewandowski.
"It is no secret that we all have a very good relationship with each other," he said on his club's website.
"The lads here are simply sensational."