AFAIK for each new civil card (white) you should remove a random card of the same type and age. (An option here: you may remove a card that you don't like instead of randomly selecting one.) In that way the number of cards in each age and their types will not change. (It opens another option: you may reveal the randomly removed card or you may keep it hidden...) Military cards should just be added to the deck.
This is how BoardGaming-Online is implemented.
Yet I have never seen anyone actually doing that in real life. People instead just add the new cards (even when playing with more than one from those small expansions). This is a minor imbalance not disturbing game play - at least in my opinion. While it makes more cards and is much simpler technically (because you just keep all the cards togather without having to take care of sorting the expansions out and so on).
would you play with all the promos cards together (i mean just adding them) => czech + 2 sets of polish + korean + spanish promo cards ?