How to import properly thai language into writer or calc ?

Good morning to all.
When typing thaï language, there isn’t any problems, in both Writer or Calc.

But when copying some text from website, MS excel,MS word then pasting (pasting special an i tested all the option i can think about) there is a problem with the sub-written voyels and the above voyels signs or accent which make the text incomprehensible.

Is there another way to cut/import thai text without this proble or is it a know bugs of Libreoffice ?