Its just logistics...unless they want to do a regionally limited release they would have to have all language VA...couldn't be cheap to do that. However, what I think(fear) they did was; the two expansions and any DLC were already in the works early! When they had the VAs in the studio to record for the original story they just did the extra readings then. So CDPR has everything they need for the main story/side quests AND for the next two years of expansions all recorded as of right now. It would only make good business sense to do it that way.
Well even if they did record everything already, we're talking of a couple hours of work for 3 (or even 2) people per localisation, provided it's CDPR and not the distributor who pays for the various localisations themselves. Depends of the contract and the clauses.
It may even be the VO companies are required to be ready for minor reworks with little to no cost added (that's what a smart company would do not to risk they cannot reuse the same VA in the same game in additinal content or if things go wrong in postproduction).
I mean, in Witcher 3's case it may be the case the have already prepped the expansions months in advance, but some games have them coming out even years later.
I don't get why so little work would cost so much, proportionally to the main game.
It's not a movie and they are not re-shoots with sets, makeup, dozens of staff involved.
Modders show us you can do a VO with a 50$ microphone in your basement.