Don't know for sure if this is the case with the C, but generally international models have EFIGS (English, French, Italian, German, Spanish) language resources in ROM while the US versions have only English. Also some European countries use a few WiFi channels we don't in the US, so there many be support for those.
I believe it isn't merely "... some European countries use a few WiFi channels we don't in the US...", but that their regulatory bodies have divided up the 2.4 GHx spectrum differently, and thus the US and European channels do not really mesh. The radios (or at least the firmware) is different, and may lead to incompatibilities when going back and forth.
Of course, when I was with Cisco, and I traveled to Europe, my WiFi laptop card seemed to work fine, so it is probably regulatory mumbo jumbo for the most part.
Geoff - the newbie