What da heck ?! When one is half-thai, and can't speak thai perfectly you go like "duh, she/he can't even make enough effort to speak/write thai flawlessly ?" or "Why did their mother/father not teach them thai language?".
Though when these ลูกครึ่ง can speak thai perfectly, but not their "second" language, it goes like "why do you even bother to be born half something if you can't speak another different language".