Yellow stains on plaster wall patches
Things can yellow for a variety of reasons, water, smoking, etc etc etc.  It really depends on the materials you are working with as well.  If the wallpaper came off that easily, water is a good guess.


What should I do?  Lots of options, I have two suggestions (see below)
Is it normal?  for a house that old, normal is a relative term, Ill go with yes
Is it moist or something like glue behind paper-like sheets?  There is likely glue of some sort, it could also be moist.  You should ensure there is nothing (a leaking pipe perhaps) behind the wall.  Its also entirely possible that there was water at some time in the past but it is no longer an issue.
Should I consider putting «placoplâte» in french (Plasterboard in english I guess or draywall) to cover all over these walls?  I wouldnt go over the walls, see my suggestions below
Is there anything I must do to ensure my walls a really dry?  Its definitely worth checking into.  If you are already patching it wouldn't hurt to cut a small hole where you suspect moisture and verify there is nothing behind the plaster.
Will it "sweat" over the paint I'll put over this or it stays only on my plaster patches?  Not sure what you mean here.  Will what sweat?


In my mind you have one of two options:


The cheaper, easier option:  Prep the walls to make them smooth and clean (in this case meaning free of dust and other things that can mess with paint).  Repaint the entire room with an oil or alcohol based sealer/primer.  Anything that is in the plaster (I am guessing this is plaster you are working with) this step will seal it in; odors, chemicals, stains...including the yellowing you mentioned.  Once you have done that, paint the walls to your taste with a latex paint.
The more expensive option:  Get yourself a dust mask and a sledge hammer and some tarps for the floor and tear the plaster out (demolition is fun, but test for lead paint first).  Replace with drywall.  This option is obviously a lot more involved.


Good luck.