Utilisant l'encodage `utf8x` au lieu de `utf8` et compilé avec `pdflatex`, le code suivant fonctionne :
\documentclass[12pt, b5paper, thai, french, a5paper, BCOR = 0pt, DIV = 15]{scrartcl}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
%\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage[dvips]{graphicx}
\usepackage[utf8x]{inputenc}
% English is the main language
\usepackage[thai,french]{babel}
% Hack into CJKutf8 package for an option clash error
\makeatletter
\@namedef{opt@inputenc.sty}{utf8}
\makeatother
\usepackage{CJKutf8}
\begin{document}
\thispagestyle{empty}
\begin{otherlangage}{thai}
{\Huge \textthai{ต้มข่าไก่}}\\
{\Large ต้มข่าไก่}
\end{otherlangage}\\
\emph{Tom Kha Gai}
Gai}}
\end{document}