Search before posting, as it was already discussed. -_-
In short - they changed the name from Russian version to Ukrainian one, for obvious reasons.

Looks like there was thread about it but been deleted. Whatever reason.
Original release and name are NA-region release from THQ. Proper name. Whatever reasons to rename (even if city name sgould be differ than game title, it is still fiction after all regardless anything else).

UPD: Oh you mean next game title name. In that case not a big deal how it is sub-titled. Original game title was by THQ. RUS/UA release didnt have a sub-title initially and just called STALKER before it is going to more titles, web-browser online game and books series. But original release - by THQ - always been sub-titled.

Chornobyl more correct. But again - fiction, sub-titles. Maybe some else arguments. Then, after all of that, not a bjg deal how its named.