And what happens when they go Commintern and get their tanks from the USSR, or Fascist and get them from Germany/Italy? Do they need to create sprites for each possibility just in case? Then shouldn't this be the case for Spain, China, etc.?
This is a non-existent problem.
You could say much the same of any of the Commonwealth countries, or even China itself. Names and artwork should be based on historical equipment where possible.
As for Portugal joining the Comintern, that is 1000% fantasy for which it would be a silly waste of time in trying to account in terms of historical equipment names and 2D art.
Prioritizing equipment historically used by the country in question for name, models, and 2D art suffices for gameplay purposes.
One could just as well say that, what if the US becomes puppeted by Japan, does it need entirely new equipment names, 2D art, and sprites? Why stop there, even? By your reasoning we must account for every possible permutation of master, subject, faction, and ideology.
This just isnt reasonable, or even desirable in light of Paradox's seemingly extremely limited resources which they put towards the development of the game, especially in light of the actual purpose of artwork in the game.
If it bothers you, then just play pretend that Communist Portugal is using T-34's instead of Cromwells or Shermans. As far as the game is concerned there is no difference aside from pure aesthetics.