The Board (in a French association) has been mandated by the General Assembly to make the decisions concerning the association. As such, it is totally in the realm of its power to delegate someone to "represent" the association at a meeting and even "make decisions" on its behalf. The only thing being, if the person makes a decision that the board is not ready to support, then the board has to live with it and be accountable in the next General Assembly.
As I pointed out, I think it's more of a cultural issue than a legal one. :)
If it helps, this is the situation in the UK (NB: IANAL, but I have spent far too much time reading the relevant legislation and various bits of official advice regarding it):
Since the UK chapter is a charity, charity law, rather than just company law, applies. It's far more strict on this kind of thing (companies can do pretty much whatever they like). The board can certainly send a representative with instructions to present the opinions already decided on by the board to the meeting and vote accordingly, there could probably be some leeway to negotiate on matters that don't affect the principles of a decision, but that's about it. There is no way an individual board member could make binding agreements on behalf of the charity, even on matters already decided by the board. Two board members, on the other hand, have far more power - the board can grant wide ranging powers (I believe pretty much all the powers of the board if they want) to a committee of two board members, and those two board members can even sign contracts and deeds and assurances and various other things I've never been able to work out the difference between on behalf of the board.
So, my advice to whoever is organising this meeting: If you want to make life easy on the UK chapter and have a productive meeting, invite at least two representatives from each country. (I suppose you could invite one from most countries and 2 from the UK if you really wanted to, but I wouldn't recommend it!)
Also, from a purely personal perspective, it's far nicer for the reps to have a friend by their side, especially for those that have limited English language skills since other people from their chapter may well be the only people they can converse with easily. And, having the responsibility for representing your chapter resting solely on your shoulders could be extremely stressful.