In short, they would have been able to understand each other as much as a modern Parisian and a Regensburger would be able to understand each other today, in other words, not at all. While France was ruled by Frankish kings during that period, that doesn't mean that the local peasants spoke Frankish (a Germanic language), and in fact by that time the Frankish kings themselves had probably also ceased to speak Frankish and instead conversed in whatever precursor to French was spoken there at the time.