\documentclass[varwidth,border=2pt]{standalone}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX}
\setmainfont{EB Garamond}
\newcommand{\startsquarepar}{%
    \par\begingroup \parfillskip 0pt \relax}
\newcommand{\stopsquarepar}{%
    \par\endgroup}
\begin{document}
\startsquarepar UN SOI-DISANT `QUIPROQUO.'\break
\noindent ---A-t-il dit, ``Qui-pro-quo!'' ---``¿Qué?"\break
\noindent Quousque tandem abutere, Catilina?\stopsquarepar
\end{document}