I have backup en-US.locPak files dating back to Sim Update 4 (1.16.2.0), and the bug is in that one. But it could very well go all the way back to .
Here are some repro steps, plus optional steps to prove the root cause.
Start Microsoft Flight Simulator in English.
Navigate to the World Map.
Click on the aircraft in the World Map menu as if you’re going to change aircraft.
Click the “All” dropdown in the upper-right corner of the screen and scroll down to the option that says “Rotorcraft”.
Expected result: The dropdown has an entry that says “Rotorcraft”.
Observed result: The dropdown has an entry that says “Rotorcrafts”. (bug – see first screenshot)
Don’t start a flight. Instead, go to Profile > My Hangar.
Click the “Change Aircraft” button on the bottom of the screen.
Click the “All” dropdown in the upper-right corner of the screen and scroll down to the option that says “Rotorcraft”.
Expected result: The dropdown has an entry that says “Rotorcraft”.
Observed result: The dropdown has an entry that says “Rotorcrafts”. (bug, same root cause – see second screenshot)
Optional steps to prove the root cause:
Shut down Microsoft Flight Simulator.
Open up fs-base\en-US.locPak in your favorite text editor (or your least favorite text editor ).
Change this line so that “Rotorcrafts” is “Rotorcraft”:
Here is proof that both instances are from the same root cause. If I change the en-US.locPak file in one place, it gets fixed in both places (see steps 8-11 below):