And the keys: (I got four of them up to now, but I found the rest in french. I try to translate it

)
Clé de Gordon:

In level 3 there's a door which opens as you end the level. Walk through it and look what happens...

Clé de Rigor-Dangerous: Put a bomb under the C of the word "ICE" in level 55, go through the vortex. Then try to reach the red button instead of going back, and then you have to be quick to reach the key (it appears as you hit the button).
--> use in level 51.
Clé des Mondes Ardus: Get to level 50 in the "Cauchemar" mode (whatever this is...)
--> use in level 82
Clé piquante: Get to level 23.1 in mirror mode
--> use in level 62
Furtok Glaciale: End the parallel dimension of level 15
--> use in level 64
Passe-partout de Tuberculoz: Beat Tuber...
--> use in level 0 (the door at the right)
Clé des cauchemars: Get to level 103 in "cauchemar mode"
--> use in level 2
Passe-partout en bois: End the parallel dimension of level 46.
--> use in level 15
Méluzzine: Seems to be a normal item (but quite rare: Coeff. 5)
--> use in level 78
Vieille clé rouillée: normal item; rare, but not as rare as the Méluzzine (Coeff. 4)
--> use in level 25
Autorisation du bois-joli: End the "paperwork" quest, then it's a normal item (Coeff. 3)
--> use in level 15.12
Clé du Bourru: End the "metallic water" quest, then it's a normal item (Coeff. 3)
--> use in level 33