Is drag the only aerodynamic force that acts upon an asymmetric body (a glider in particular) travelling through the air? It is known that for a symmetric body, like a sphere falling through the air, the only aerodynamic force that acts upon it is the drag that points always in the opposite direction relative to its speed.

Now, an asymmetric body traveling through the air, I guess, will have a drag that makes an angle different from 180 degree with its speed.

As a glider is just an asymmetric body, it means that the only aerodynamic force acting upon it is the drag that is conventionally decomposed in what we call Lift and Drag.

Is it true that Lift and Drag applied to gliders or airplanes are just a convention, two forces that in reality do not exist, and there is only a True Drag that makes an angle of a certain value with the speed of the aircraft?
You are right, lift and drag are just convention. But the aerodynamic forces are usually grouped into one total force named "total aerodynamic force" ("résultante aérodynamique" in french).

As explain in how it flies:


  
  drag is a component of the aerodynamic force, namely the projection onto the direction parallel to the relative wind.
  lift is another component of the aerodynamic force, namely the projection onto the two directions perpendicular to the relative wind. 
  


This convention applies to any aircraft.

For further reading, see the section about forces of how it flies.