Okay, this is a little theory of mine,
I did not try it yet, and since I'm not a math guy at all, I could very possibly went wrong with my reasoning.
So...
IF, and only IF center chanel (audio english) and (audio french) share the same EXACT mix (and it is rarely, if never, the case... so I guess all this theory means nothing in the end, lol) then:
Step 1: (audio english) + inverted audio french) = English voices mixed with inverted french voices
Step 2: (audio english) + (English voices mixed with inverted french voices) = (audio french (with french voices being inverted)) mixed with (english voices at volume level X2)
Step 3: (audio french (with french voices being inverted)) mixed with (english at volume level X2) + (audio french) = (audio english (with voices at volume level X2)
Step 4: (audio english (with voices at volume level X2) + (inverted audio english) = .... English voices at original volume level.
Step 5: (audio english) + inverted (English voices at original volume level) = (audio english) minus english voices...
EDIT: All this reasoning is based on the supposition that an inverted audio removes half of the same original audio if its level is twice the same. And this is not a given.
RE EDIT: mmmm step 3 obviously double the music.... I must return to my reasoning, lol.