The French currently drive on the right so they are left hand drive but illustrations I have seen of WW1 trucks have the driver seated on the right.
They drove on the right even in WWI, because we can blame Napoleon for that (who else?) -- the law is from 1792.
However, as has been pointed out, the automotive industry was still in its infancy and driver positions customized for local traffic regulations pretty much didn't exist. And in wartime if it worked, it got used damn the regulations.
Even today, it's usually not illegal to drive a vehicle with the driver position on the "wrong" side -- just inconvenient and somewhat less safe.