First of all, you have to know that in France, pressures are measured with millibars instead of mmHg. The pressure of a standard atmosphere equals 1013 mbars.
For official regulations, you should of course check the french AIP. I can't garantee that no mistakes were made here. This is NO official documentation.
Unless you are in a controlled airspace, with a transition level and altitude, whose rules are to be followed like in other countries, there is a special rule about using and reporting one's height or altitude.
There is a surface called "S" surface, covering the whole French territory. It is now 3000 ft AGL. Please note that this definition changed in january 2007 (before, it was 3000AMSL or 1000AGL, whichever the highest.)
When you fly ABOVE the "S" surface, you MUST use
Flight Levels., and set your altimeter using 1013 mbar, whatever
the real pressure.
This surface is also very important because above it, you must use flight levels depending on your flight rules and the heading of your route.
If you are VFR, you may use only FL finishing with a 5 : 35, 45, 55 ,65, 75, 85....
If your route heading is between 0 and 179, you must use "odd" levels : 35, 55, 75, 95, 115...
If your route heading is between 180 and 359, you must use "even" levels : 25, 45, 65, 85... (FL25 appears when the real pressure is higher than 1032 mbars.)
VFR ON TOP : Above the "S" surface, provided your aircraft is equipped with at least a VOR receiver, you may fly "on top". It means that you may fly above clouds, even a layer of clouds. But beware. If you're not a IR pilot, you're not allowed to enter any cloud. Thus, to fly on top, you must have found a hole between clouds enabling you to climb over them, and you must be SURE to find a hole large enough for you to descend when you're near your destination. If you take off under a clear sky, but with a forecast of overcast wheather at your destination, don't fly ontop, because you will probably be unable to go down safely then. If I may give you an advice, don't fly on top if the forecast at your destination is more cloudy than 4/8. Don't fly on top in a single engined aircraft over mountaneous ground, if you know that mountains'top might be in the clouds. Imagine what would happen if you had to do a forced landing.
When you fly UNDER this surface, you are free to choose you flight altitude. You're free also to use either QFE or QNH to measure and report your position. These letters groups are remnants of an old three letters code used in aviation. We still use them a lot. Under this surface, you MUST be in sight of the ground. No "on top" flight under the "S" surface.
VMC depends also on your position above or under this surface.
To be 2000ft QFE means you are 2000 ft above airport level.
Your reference for setting up your altimeter is the real pressure
at the airport level.
To be 2000 ft QNH means that your altitude (above medium sea level) is 2000 ft. Your barometric reference is the real pressure at sea level. You compute it by adding to the real pressure measured at ground level a fixed value, depending on the altitude of the place where you are.
1 mbar corresponds to 28ft. If your airport is 560ft high, the difference between sea level and ground level pressure is 20 mbars.
Imagine you are on a apron, on the ground, but at an airport
altitude of 1100 ft. The real pressure on the airfield is 980
mbar. You set your altimeter using this pressure. It will indicate
0 ft. When you take off, you will read you height above the ground
of the airfield. If you report your position on the radio, you
must precise that the height you give is "Fox-Echo".
For exemple, : "passing overhead the runway, 1500ft fox echo".
If you want to use the QNH, which is the real altitude, just add 39 mbar to the ground pressure before you set up your altimeter. (1100ft corresponding to 39mbars). When you're on the ground, with a reference of 1019mbars, your altimeter indicates 1100 ft. In the same position as previously, you would say : "passing overhead the runway, 2600ft (QNH)".
More and more pilots use only QNH now, and you're advised to use it as well. But you may still hear QFE reports. If you use QNH, you don't have to say it explicitely. On the contrary, you must add QFE in your message if you use this reference. If you hear a radio message with an altitude expressed with no barometric reference, it means that the pilot uses QNH. If he states an AAL, he has to add explicitely QFE.
Metar give both QNH and QFE pressure references.
Be carefull. If the pressure is lower than 1013, your real altitude is lower than the altitude indicated by your altimeter when you fly above the "S" surface (since you set it up using 1013 instead of the real value). If you choose a low flight level, you may be dangerously close to the ground whereas your altimeter indicates a reassuring value. When visibility is poor, this can be VERY dangerous, and several pilots already lost their lifes in such accidents.