Please please please have a real look on how Debian Installer works
before throwing ideas in the wild, if possible.
The above case is perfectly dealt with by d-i. English speaking
Canadians will choose "English (other countries)" at first screen,
then "Canada" at second screen (which will then list all countries for
which a valid en_XX locale exists). French-speaking Canadians will
choose "French (Canada)" at first screen and will indeed never see the
second screen.
Same for Dutch/French/German speaking people in Belgium,
German/French/Italian speaking people in Switzerland and so on.....