Advanced system utilities to manage, troubleshoot, and diagnose Windows and Linux systems and applications.
As a workaround launch RDCMan.exe with the -mstsc parameter.
If you are using the Microsoft Store version modifying the icon is not possible. Locate the installed copy of the executable (mine was C:\Users<user>\AppData\Local\Microsoft\WindowsApps\RDCMan.exe) and create your own shortcut that includes the -mstsc parameter.
Or alternatively download the portable version as the .ZIP, extract it to a path of your choice, and build your own shortcut if desired to it so that it uses the -mstsc paramter. E.g. I place mine at C:\Program Files\Microsoft\Remote Desktop Connection Manager\RDCMan.exe and put my shortcut in the All Users start menu so it acts like any regular installed program for everyone on the machine.
Here I use Canadian English with a US keyboard layout. As of v3.22 it picks Canadian French for the remote session when using the new engine. With -mstsc to force the old engine it correctly uses Canadian English.