Since I have learnt of this practice, I thought that placing ashes on Ash Wednesday is practiced worldwide, at least within Roman Catholic Church. But now I have been on a Mass (from first Lent Sunday, when ashes may be placed for those who didn't receive this sacramental on Ash Wednesday) attended by some Portuguese. They seemed puzzled when they saw the priest placing ashes on other worshippers - they apparently didn't know the practice.
I checked Czech (I'm Czech) and English wikipedia, and they both highlight the practice of placing ashes. I don't speak Portuguese, but Portuguese wikipedia has no ashes-themed picture and from the little I understand it seems that this practice is not even mentioned there. Anyway, Portugal is unlikely to be the only country not observing imposition of ashes. English wikipedia gives a list of churches observing Ash Wednesday, but not of countries where the practice of placing ashes is (or is not) observed in Roman Catholic Church. Is there any such list?