% arara: pdflatex
\documentclass{article}

\usepackage[main=german-de,USenglish]{babel}
\usepackage{regions2}

\newcommand{\test}[1]{%
 \section{#1}
 \selectlanguage{#1}%
 \SetCurrentTrackedDialect{#1}%
 Language: \CurrentTrackedLanguageTag.

 \unitname: \weightunit, \lengthunit, \currencyunit.
}



\begin{document}
\test{USenglish}

\test{german-de}

\end{document}