from https://entreresource.com/how-to-download-facebook-messenger-conversations. and https://stackoverflow.com/a/68237082/11501916

Golang - Decode Facebook Messages

type Input struct {
	SenderName string `json:"sender_name"`
	Content    string `json:"content"`
	Type       string `json:"type"`
}

func main() {
	//bs, _ := ioutil.ReadFile("./input.json")
	bs := `{
  "sender_name": "Kreang \u00e0\u00b9\u0080\u00e0\u00b8\u0081\u00e0\u00b8\u00b5\u00e0\u00b8\u00a2\u00e0\u00b8\u0087",
  "timestamp_ms": 1638599904881,
  "content": "\u00e0\u00b8\u00aa\u00e0\u00b8\u0099\u00e0\u00b9\u0083\u00e0\u00b8\u0088 \u00e0\u00b8\u0082\u00e0\u00b8\u00ad\u00e0\u00b8\u00a3\u00e0\u00b8\u00b2\u00e0\u00b8\u00a2\u00e0\u00b8\u00a5\u00e0\u00b8\u00b0\u00e0\u00b9\u0080\u00e0\u00b8\u00ad\u00e0\u00b8\u00b5\u00e0\u00b8\u00a2\u00e0\u00b8\u0094\u00e0\u00b9\u0084\u00e0\u00b8\u0094\u00e0\u00b9\u0089\u00e0\u00b9\u0084\u00e0\u00b8\u00ab\u00e0\u00b8\u00a1\u00e0\u00b8\u0084\u00e0\u00b8\u00a3\u00e0\u00b8\u00b1\u00e0\u00b8\u009a",
  "type": "Generic",
  "is_unsent": false
}`
	input := Input{}
	json.Unmarshal([]byte(bs), &input)

	var uint8List []uint8
	for _, v := range input.SenderName {
		uint8List = append(uint8List, uint8(v))
	}
	fmt.Println(string(uint8List))
}