public static String getCountryNameByIsoCode(String isoCode){
    if(isoCode == null) {
        return null;
    }
    
    Schema.DescribeFieldResult fieldResult = User.Countrycode.getDescribe();
    List pickListValues = fieldResult.getPicklistValues();
    
    for( Schema.PicklistEntry pickListEntry : pickListValues) {
        // pickListEntry.getLabel() returns the country name
        // pickListEntry.getValue() returns the country code
        if(pickListEntry.getValue().toLowerCase() == isoCode.toLowerCase()) {
            return pickListEntry.getLabel();
        }
    }
    return null;
  }