Bug using breqn and babel

Question

I've hit a bug that I cannot fix (no warnings/errors in the logs).

Here is a mwe:

\documentclass{article}
\usepackage[english,french]{babel}
\usepackage{breqn}

\begin{document}
  \[\{A, B, C\}\]

  \selectlanguage{french}
  \[\{A, B, C\}\]

  \selectlanguage{english}
  \[\{A, B, C\}\]
\end{document}

I use the following command to compile:

$ latexmk -lualatex bug.tex

The output is as follows, the commas are mapped to a \Gamma symbol !?

Does anyone have a clue on how to fix this ?

$ lualatex --version
This is LuaTeX, Version 1.07.0 (TeX Live 2018)
$ latexmk --version
Latexmk, John Collins, 17 Jan. 2018. Version 4.55

Answer 1

The babel-french module does \std@math@comma upon entering and also exiting French language blocks.

This resets the mathcode for the comma to the standard, but breqn wants that the mathcode stays "8000.

Simply make \std@math@comma to do nothing.

\documentclass{article}
\usepackage[english,french]{babel}
\usepackage{breqn}

\makeatletter
\let\std@math@comma\relax
\makeatother

\begin{document}

\[\{A, B, C\}\]

\selectlanguage{french}
\[\{A, B, C\}\]

\selectlanguage{english}
\[\{A, B, C\}\]

\end{document}

You should be aware that breqn is not really compatible with several package and is better not used in serious typesetting tasks.

Answer 2

For lualatex use

\documentclass{article}
\usepackage[english,french]{babel}
\usepackage{breqn}
\usepackage{unicode-math}
...