OK well some 10 years ago when I first applied for my HGV here I did declare it, have declared it every exam after and never had a problem. We have a medical of about €25 per time and all that you pay for on top of that is the photographs. Apparently if there is a problem with your diabetes then there may be inquiries, certainly if you are a new driver with pre existing diabeties as there is nothing to go on, however, if you already have a licence and then become diabetic you only have to notify if you have a problem and your doctor will tell you that.
However, I spoke actually to my diabetes doctor about this sort of thing and it seems that as diabetics are FULLY covered for medical disbursements that the person that is to gain a permit is supposed to deal with this at the time of a normal periodic medical and she will inform the doctor assigned to do the HGV or whatever medical. It also seems to be on how bad ones problem is. As I no longer wish to maintain my HGV after next year my licence is valid for about 21 years through out the EU (DVLA Read) However any licence issued from this year in France has a 10 year limit and after 70 a regular medical (free incidentally as it should be in the UK according to the DVLA!!)
If the authorities are not careful with this sort of thing then many people will just not bother, no insurance, no licence, and in an exported car! Lets face it there is only so much people can take, if for example I was not allowed to drive then frankly I would not be able to live, no food, no medical, no drugs. We have no public transport around here, so it means the state wherever, will have to arrange housing, no argument we cannot spend loads of money on for example medicals, the figures you quote is a weeks money after things like house tax, house insurance, water, electric, possibly more.
We do have a thing here called sans permis, but at 30MPH or less a 1 hour journey to the local shops, doctor etc. is a bit silly!
It does not surprise me that many french do not know how to go about the licence thing, we had bum information on them, it has not changed, its the same with tax and loads of other things. We had to straighten the DVLA out on driving licences, an EU licence is valid in the UK until its expiry date or 21 odd years which ever comes first for residents of the UK AND any other EU country. Incidentally IF the UK start putting a Union Flag on to Driving licences and a chip then the licence will NOT be valid in Europe! It will not be harmonized!
Go and get a Maltese licence now, its valid for the 21 odd years and easy to get with an exchange! OK for car and bike! Theirs are supposed to change next year!!!