\documentclass{article}
\usepackage[english]{babel}
\babelprovide[import]{thai} % Remove main if the main language is english
\babelfont[thai]{rm}{FreeSerif}
\begin{document}
Hello.
\begin{otherlanguage}{thai}
สวัสดี
\end{otherlanguage}
\end{document}
Fails only with XeTeX: