Well, as I said, we need to either pick one form and stick with it throughout, or offer both (or more than one) form in the game. Most big games, office suites, and other programs that Ive seen have at *least* two - which tells me that United States English and Euro/British are the most common (though I've seen some software that have several more choices, including some more obscure forms).
Trying to argue or "prove" one form to be "better" than another is totally pointless. No matter how hard one tries to rationalize it or back it up with any supporting argument, someone else can do the same to push it the other way.
If we resort to such back-and-forth debate we might as well argue over the colors red vs. the color blue, and which color in general is "better" than the other.
"Red is better! It is stronger and can look brighter!"
"No, blue is better, it isn't harsh like red is!"
...etc., etc., etc., onwards into total absurdity...
The one difference, where such a comparison doesn't apply, is that with colors the two can be mixed into purple, which is a valid color. In this case though, with the language issue, there isn't any "purple" mix that would really work, it should either be consistent with one or be split for the end-user to choose.