It looks like you did your homework. Good job. This is quite a mystery.
Apparently, as you've found,
"C:\Program Files\Windows Defender\MSASCui.exe" -hide
is the correct command, and the French command is simply ignored, whereby
Defender is not hidden at startup.
Running the English command in RUN, from your desktop, should not be any
different that having the registry run the same command at startup.
This appears to be a bug that Microsoft should know about. If you're up for
the challenge, you could try this:
Open Notepad, and copy the following (between the dashes)(i.e, don't include
the dashes):
-----------------------------------------------------------------------------------
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]
"Windows
Defender"=hex(2):25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,\
00,69,00,6c,00,65,00,73,00,25,00,5c,00,57,00,69,00,6e,00,64,00,6f,00,77,00,\
73,00,20,00,44,00,65,00,66,00,65,00,6e,00,64,00,65,00,72,00,5c,00,4d,00,53,\
00,41,00,53,00,43,00,75,00,69,00,2e,00,65,00,78,00,65,00,20,00,2d,00,68,00,\
69,00,64,00,65,00,00,00
-----------------------------------------------------------------------------------
Paste it into Notepad, and save the file as defenderstart.reg (it should not
have a .txt extension). Close Notepad. Double-click on the file
defenderstart.reg, and allow the merge when asked.
Restart your computer to see if this works. If there are the same problems,
run update / repair again in Add/Remove, which will put things back where
they started. Post back, and in the meantime I will try to figure out how to
report this problem to Microsoft.