Sorry i did respond to this but its disappeared.
I’m grabbing both a French Windows 10 Install disk as well as a French Canadian Windows 10 Installation disk to try and see if i can replicate what you’re trying to do via MDT.
Can you please let me know what it is that isn’t working? There is obviously something that isn’t right when you install so what is it? Is it the UILanguage that is wrong, is it the KeyboardLocale that’s wrong, is it the UserLocal thats wrong or is it the InputLocale that is wrong?
A French/French Canadian disk should install by default with French as the language. I would assume that French Canadian would do the same.