Compatibility issue between chemmacros and polyglossia when using `russian` language setting

Question

I want to use two packages chemmacros and polyglossia together in my document, and for some reason I don't want to use mhchem and babel etc. instead of them. Here are some simplified examples.

Everything behaves fine when setting the language to english.

\documentclass{article}

\usepackage{polyglossia}
\usepackage{chemmacros}

\setotherlanguage{english} % Produces correct results under language `english'.

\begin{document}

\ch{H2O}

\end{document}

However, when I switch the language to russian, \ch{H2O} seems to produce unexpected results, resembling \operatorname{ch}H2O.

\documentclass{article}

\usepackage{polyglossia}
\usepackage{chemmacros}

\setotherlanguage{russian} % Produces incorrect results under language `russian'.

\begin{document}

\ch{H2O} % An error `Missing $ inserted.' is reported, and the result is incorrect.

$\ch{H2O}$ % No error is reported, but the result is still incorrect.

$\operatorname{ch}H2O$ % As a comparison.

\end{document}

How can I solve this problem and make chemmacros behave correctly when the document language is russian?

I am using XeLaTeX as the compiler and TeX Live version 2023.

Answer 1

By default, the russian module for Polyglossia defines some operators according to Russian traditions, in particular \sh and \ch for the hyperbolic functions.

Of course, having \ch defined conflicts with chemmacros.

If you don't need them, you can just exclude them at loading time:

\documentclass{article}

\usepackage{polyglossia}
\usepackage{chemmacros}

\setmainfont{Libertinus Serif}

\setmainlanguage[mathfunctions=false]{russian}

\begin{document}

\ch{H2O}

\end{document}

What happens? chemmacros defines its own version of \ch at begin document, whereas polyglossia defines the meaning of \ch for the hyperbolic cosine operator at the point when it switches language, which happens later. So polyglossia wins.

If you want to keep the Russian-style operators, you need to change \ch from chemmacros (actually chemformula) to something else:

\documentclass{article}

\usepackage{polyglossia}
\usepackage{chemmacros}

\setmainfont{Libertinus Serif}

\setmainlanguage{russian}

\AddToHook{file/chemformula.sty/after}{%
  \NewCommandCopy{\CH}{\ch}%
}

\begin{document}

\CH{H2O}

$\ch x=(e^x+e^{-x})/2$

\end{document}