\usepackage{polyglossia}
\setdefaultlanguage{english}
\setotherlanguage{latin}
\begin{document}
\begin{latin}
Lorem ipsum dolor
\end{latin}
\end{document}
----------
Try:
\documentclass{article}
\usepackage[english,latin,base]{babel}
\usepackage{polyglossia}
The `base' option in babel just preloads the patterns. I'm aware
this change in babel can be somewhat annoying, since it's based
in a quite different model from the previous one (I tried hard
to provide both, but I didn't manage it).
Javier
