Stuck with the very same problem. Found that clearing Performers
first makes it work as intended:
using(TagLib.File tlFile = TagLib.File.Create(newFileName)){
//tlFile.Tag.Performers = new []{translateDict[author]}; //doesn't work
tlFile.Tag.Performers = null; //clearing out performers
tlFile.Tag.Performers = new []{translateDict[author]}; //works now
tlFile.Save();
}