polyglossia: how to set `\frenchspacing` for all languages, everywhere?

Question

How can I disable the double space between sentences for all languages in the following MEW?

\documentclass{article}
\usepackage{fontspec}
\usepackage{polyglossia}

\setmainlanguage{german}
\setotherlanguages{english}

\parindent=0pt

\begin{document}

This. Is a sentence in the main language (german). 

\begin{english}
This. Is a sentence in english environment.
\end{english}

This. Is a sentence in the main language (german). 

\textenglish{This. Is a sentence in textenglish [double space between
sentences].}

This. Is a sentence in the main language (german). 

\end{document}

The strange thing is that a nonfrenchspacing occurs only in the \textenglish inline version of english text, not in the block version.

Answer 1

% !TeX TS-program = xelatex

\documentclass{article}
\usepackage{fontspec}
\usepackage{polyglossia}

\setmainlanguage{german}
\setotherlanguages{english}

\parindent=0pt

\appto{\textenglish}{\frenchspacing}


\begin{document}

\frenchspacing

This. Is a sentence in the main language (german). 

\begin{english}
This. Is a sentence in english environment.
\end{english}

This. Is a sentence in the main language (german). 

\textenglish{This. Is a sentence in textenglish [double space between
sentences].}

This. Is a sentence in the main language (german). 

\end{document}