'The Germans,Spaniards and Poles' This is OK
'They fought against Germans'- This doesn't work as it suggest they fought against Germans in general, not a specific group at a specific time.
The Americans say they are the peacemakers not the warmongers.
The are the only peacemakers
The Americans say they are peacemakers not warmongers.
They are peacemakers among others
Americans say they are peacemakers not warmongers etc.
This is Americans in general, while the first two suggest the government, authorities, etc
'The mother and father (son)' or 'The mother and the father (the son),'
You can use either form
Student or Learner