is it only the french that don't stand any fair critic ?
Generally, I think that the French reject critic (fair or unfair) more than others. One fundamental reason is that, in France accepting a critic is acknowledging that you're wrong and in France you can not be wrong.
For years, the education and cultural system have programmed the French kids to be right: have the right answer, have the academic knowledge, have the perfect mark... And that's serious programming that starts at 2 or 3 y.o. and repeats every day.
Failure (being wrong) is not a possibility since it means being judged (by teachers, peers and family), rejected and ultimately not being loved (what the French crave for and never really got). Being right is being loved.
This obligation to be right is so strong in this country that it even superseded Truth. Thus a lot of French (in particular the Parisian elites and the likes) have developed tremendous oratory, logical, rhetorical skills in order to end up being right (being loved) even if what they claim is not true.
Now critic made against France by foreigners triggers something else in the French: their strong identification to their country. Why are the French so identified to France? My guess is that as children they were prevented from developing a true self, self-confidence, tolerance because of an individualistic and competitive educational system that is based on negative reinforcement (sanctions) and only spares the best, the most obedient, the most analytical ones.
Devoid of their true being and of self-confidence the French had to find substitutes. One of this substitutes is appearances. Overcompensating their fears, their uneasiness, their sadness they became master at creating positive appearances (speech, clothes, attitudes...) to hide a desperate inner being. They are not happy or peaceful but, at least they look like they are.
Another substitute is the nation. Unable to feel genuine self-confidence, they overcompensate and hide their fear in an exaggerated pride for their country (chauvinism). In their childhood French kids have been brainwashed to follow rules, to pursue the same goal, to speak and think the same way to share the same values while any creativity, difference, emotion was being repressed. This leads to adults exhibiting very strong conformity and power distance (respect for authority) while being devoid from genuine personality, personal goals. Thus, the self worth of the French lies in their country, any critic against their country is viscerally perceived as a personal critic. Hence the negative reactions.
Interestingly not only the French consider France as a great country, a lot of foreigners (particularly the ones who don't know France very well) share this opinion. Several causes can explain this wrong perception. First, the French are so convinced that their country is the greatest that they sometimes manage to convince others, second the French are so good at appearances (VS being) that they created very beautiful things (building, monuments, etc.), third centuries ago France was a leading empire and might still benefit from this aura (Stockholm syndrom that you can notice in some ex French colonies), fourth the landscape in France is exceptional and surely gives a very good impression to people who only see France through sightseeing and/or pictures.
Now, the French react even more negatively to critics against their country when they come from foreigners. As mentioned above France exhibits one of the highest conformity level amongst all nations. So, the foreigners (even when they don't criticize) are a living threat against the very foundation of the French mentality. The French conforms to a very specific set of values (the French culture), this values define how the French speak, do and perceive themselves. By definition a foreigner has a different culture, a different set of values, he is a living proof (especially if he is happy) that the French culture is not at all the best, that there are many other ways of living a good life.
If this is true, then the French have sacrificed their true self, their dreams, their hopes, their feelings to a culture that was not even better than other ones. It's a double loss, they didn't reach this cultural excellence they were promised and they lost their true self on the way.
An apparent paradox of the French is their strong power distance / conformity and their apparent tendency to rebel. Well, again the chasm lies between the fundamentals and the appearances. Indeed in appearances the French are rebels, immediately one can think of resistance during WWII, the 1789 revolution, the frequent demonstration, the tax evasion, the sexual freedom.
But when you scratch beyond the surface you see that during WWII most French were not in the resistance and a majority was collaborating with the Nazi regime. Same for the French Revolution were a handful of Parisian elites that established a regime (the terror) that was worse than the previous ones, demonstration are part of the French culture, participating to them is conforming to this culture, it's not rebelling especially when demonstration are about petty problems and don't lead to any concrete improvement.
The same applies to tax evasion. Sure a a lot of them cheats on income tax but most of the tax load comes from other taxes. 70% of the salaries are taxed, VAT is almost 20%. Alcohol, fuel, cigarettes reach up to 80% tax. By cheating a little bit on income tax the French maintain the appearance, the illusion that their are independent, rebels, free thinkers while the Truth is that they pay a lot of taxes. Meanwhile the elites, the ones who make them pay heavily, pay almost no tax (the average tax rate for the 40 biggest French companies is 8%!)
Now, sex in France is an interesting topic. In France having a mistress/lover is considered as positive. Going to swingers night club is considered as positive too. There are porn movie on the main cable channel, naked women on magazines, beaches, ads. The French call it "sexual liberation". But is it true? If you think about it by promoting all this free sex thing, the French again chose the appearances (pleasure) while sacrificing the substance (true love). The same way they chose the appearance of being right while sacrificing the substance of approaching the Truth. The same way the chose they appearance of being beautiful (clothes, perfumes, cosmetics, attitudes...) while sacrificing the substance of being beautiful inside (peaceful, confident, empathic...)