Erm, I'm sure.
When a Pokemon a captured, it takes the name it's supposed to have from the language file, and is saved in its data.
So let's say if you have an unnicknamed Pokemon from your save in Fre, and decided to change your game language to eng through the hack mentioned above, the existing Pokemon will remain in Fre.
because it doesn't re-reference upon every reload.
They only re-reference like when it's evolve, or other special circumstances.
With the hack above, your game will think that "Charizard" is the French name, cause you hacked the French names to use English names.
Thing is, when you connect to a server to trade, battle or whatever, the server might not recognise the discrepancy, since it'll may just compare name to your cart directory (which you hacked so it'll "seem the same").
Whatever the case it, it'll still be an illegal combination you may wanna avoid, cause we don't know for sure how the server runs it validation (it could validate against its own data, rather than the games data, because of such hacks)