From Wikipedia, there are channels from 1 to 13 on 5 MHz spacing (followed in Japan by a 14th channel spaced 12 MHz up from 13, but you won't get that in any case), with substantial overlap. Each channel has a width of +/-11 MHz at 30dB, and +-22 MHz at 50dB.

In the US, channels range from 1 to 11. That means the top channel in (most of) Europe, 13, is 10 MHz from the highest reachable channel, 11. So some reception, at short range, should be possible, even for channel 13. Expect troubles, but it should work. This is the same basis as the strange channel-hopping order of Kismet; since the channels overlap, you can get some packets while on the wrong channel.

Spain allows only 10 and 11, so no prob; France uses 10-13, meaning around half the APs there will suffer problems.

Disregard this:
If you're going to France, especially, you might try to get some EU firmware. I don't know that there is any, but you should check.

Edit: Didn't see Milhouse's post regarding single firmware.