If you don't have the nationality, you don't have to serve as a miltary. If you are born here, and have Paraguayan nationality, and get 18, you have to serve, unless you go study, to become police officer, for instance.
For foreigners, residents can be have the duty to serve in the army, in case of war, but not as military, but in supportive functions.
As far as I understood in the ley (law) Paraguaya