sogar mit Overleaf, was ist da los?
\documentclass[parskip=half]{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[british, american]{babel}
\begin{document}
\hsize=3mm
american:
\hspace{0pt} permanent result comparability nevertheless between
\selectlanguage{british}
british:
\hspace{0pt} permanent result comparability nevertheless between
\end{document}