MWE:
\documentclass[11pt]{book}
\usepackage{paracol}
\begin{document}
\begin{paracol}{2}
\switchcolumn[1]* \section*{Heading in Language 1}
\switchcolumn[0] \section*{Heading in Language 2}
\begin{enumerate}
\switchcolumn[1]* \item Text in language 1
\switchcolumn[0] \item Text in language 2
\begin{itemize}
\item \switchcolumn[1]* Why does this spill over? Shouldn't it wrap?
\item \switchcolumn[0] Why does this spill over? It shouldn't go into margins.
\end{itemize}
\switchcolumn[1]* \item More text in language 1
\switchcolumn[0] \item More text in language 2
\switchcolumn[1]* \item Yet more text in language 1
\switchcolumn[0] \item Yet more text in language 2
\end{enumerate}
\end{paracol}
\end{document}
Output:
How do I change the column width of the itemize environment and prevent it from spilling over?