ICAO is a United Nations agency and as such most members of the UN, including Eastern Block countries. It issues recommended practices for civil aviation worldwide which contracting states are supposed to accept, and where they don't, they publish a list of those recommendations which they don't follow. The USA's FAA has the largest list of differences from ICAO recommendations.
I think that the current ICAO contracting states number 188, including Russian Federation, Poland, Romania, Nigeria, USA, Slovakia, Uzbekistan, Turkmenistan, Serbia and Montenegro, Croatia and many other former Eastern Block countries. The full list can be found at:
ICAO Contracting States
If you're going to work for CHC GO they don't much like JAA or FAA licences because they don't have many examiners for those jurisdictions. If you're working in some African countries for them, they'll get you an SAA licence as the aircraft are on the ZS register. In the case of somewhere like Nigeria, they'll send you to Canada to get your Canadian licence and then keep that current for validation. Be warned that you may find it difficult to keep a JAA licence currebt if you work in Nigeria for CHC as they won't normally pay for you to keep your JAA licence current. This has led to a number of pilots leaving them.