Hey fablagrenouille
It looks great with your code, though I dont understand the french 
I am curious how you get to turn different motors with different ID's?
Also I tried your code, but I get some errors. Since I am new to Arduino I am not sure if I there is something I need to add for it work?
Errors in short:
sketch_jan07a.ino: In function 'void loop()':
sketch_jan07a:44: error: incompatible types in assignment of 'int' to 'byte [20]'
sketch_jan07a:59: error: invalid operands of types 'byte [11]' and 'int' to binary 'operator>>'
sketch_jan07a:76: error: incompatible types in assignment of 'int' to 'byte [20]'
sketch_jan07a:85: error: call of overloaded 'println(byte [20], int)' is ambiguous