Who chooses the players for the NT matches? I have a midfielder who is better than other NT argentinian midfielders, but he doesn´t play NT matches.

The players who play in a national teams were selected at the start of the season. At that point your league may not have been created yet. Or perhaps you created the player during the season?

The national selections do not change during a season. Next season your player will be considered when the new national team for that season is selected.

Exactly, I was talking about a player I've created during the season.

Thanks!