\documentclass{article}
+\usepackage{lltjext}
\usepackage{luatexja-fontspec}
\setmainjfont{IPAmjMincho}
\setmainfont{Crimson}
abc๐ใใ๐ฌใ๐ฐใใ๐ธใ๐ฟใ๐
ใ๐ใ๐\\
abc๐ใ๐ใ๐ ใ๐กใ๐ขใ๐ฃใ๐คใ๐ฅใ๐ฆใ๐งใ๐จใ\\
This Latin text should be rendered in crimson.
+
+\parbox<t>{20\zw}{%
+abc๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐\\
+abc๐ใใ๐ฌใ๐ฐใใ๐ธใ๐ฟใ๐
ใ๐ใ๐\\
+abc๐ใใ๐ฌใ๐ฐใใ๐ธใ๐ฟใ๐
ใ๐ใ๐\\
+abc๐ใ๐ใ๐ ใ๐กใ๐ขใ๐ฃใ๐คใ๐ฅใ๐ฆใ๐งใ๐จใ\\
+This Latin text should be rendered in crimson.
+}
+
\end{document}