As far as I know (NA) or other keys will work on EU/PL account, in ex. it works with Front Mission Evolved or keys gifted by people in Russia so we can try. Probably only loss is that Polish translation will be missing, but playing in full eng is fine to me (they translated Polish game from english language back to polish so end result is funny if not bizarre with stuff as names for weapons). ;-)
There are usually 3 regions for Steam games. NA, RU/CIS and RoW (Rest of World). Some games use all 3 (XCOM:EU for example), others use only RU/CIS and RoW. But a NA key will rarely work in Europe, since that means it's a RoW key, not a NA one.
RoW keys should be activatable worldwide though.