Babel seems to break letterspacing of soul package

Question

This is a MWE of a book I'm working on.

\documentclass[10pt,twoside]{book}
% \usepackage[ngerman]{babel}
\usepackage{blindtext}
\usepackage{soul}

\usepackage{fancyhdr}
\pagestyle{fancy}
\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\fancyhf{}
\fancyfoot[LE]{\so{\MakeUppercase{Book Title}}}
\fancyfoot[RO]{\so{\MakeUppercase{\leftmark}}}
\fancypagestyle{plain}{\fancyhf{}}

\begin{document}
\chapter{My Chapter Title}
\blindtext[11]
\end{document}

This is essentially what I want to have. Except that it ceases to work once I remove the comment in front of the second line. (Note, this is only about the footer on odd pages!) Somehow, the babel package seems to break the letterspacing of soul. What to do?

Answer 1

With babel, \leftmark produces a quite complicated sequence of commands and \so gives up very early.

You can use microtype instead.

\documentclass[10pt,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{blindtext}
\usepackage{microtype}

\usepackage{fancyhdr}
\pagestyle{fancy}
\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\fancyhf{}
\makeatletter
\fancyfoot[LE]{\textls{\MakeUppercase{Book Title}}}
\fancyfoot[RO]{\textls{\MakeUppercase{\leftmark}}}
\fancypagestyle{plain}{\fancyhf{}}

\begin{document}
\chapter{My Chapter Title}
\blindtext[11]
\end{document}