In my experience most French and Italian players play G/C (and occasionally C/F) while in the Netherlands they all have C / F instruments with a retuned button on the inner row to allow easier cross-rowing.

All of which means it can be an absolute bugger to teach melodeon in a foreign country, so I've some sympathy with the French chappies. You might argue that they could have arranged to borrow D/Gs, but since they tend to play their C/Gs at the upper end of the fingerboard, it would come out horribly shrill if they used the same fingering on a 'damned digibox'!

I suspect Gallic tongues may have been slightly in cheek as well...