Question: Language

Yes. So far as I've seen, the MacOS is identical in every country--it just depends on what language you select as the default (providing you didn't select not to install the language when you installed the OS).

The only difference about install is what language the login screen is in, which I believe is determined by what language you used in the installer. Pretty sure there's a trick for changing that after the fact, too, or just do a reinstall and pick French if you want a French login screen.

I've set up friends' US-bought computers to run as if they were Japanese, and everything is perfect other than the keyboard (which is of course US-standard rather than a Japanese keyboard).