Can I decide which country I work in?

MSF places you on assignment based on the needs of MSF in the field and your skills. If you are able to speak French a wider range of projects are open to you. For security reasons certain nationalities cannot work in certain countries. South Africans are generally accepted everywhere.
Expats do have a final say in accepting or rejecting a mission but are expected to be open to a wide variety of locations and activities.