Not writing manytime the language switching in the auxiliary files if not required.

[maieul]

Oups, sorry, message was send before I write it !

So my problem start here https://github.com/maieul/ledmac/issues/887.

Having a talk with @u-fischer on SE, she pointed the fact that

\documentclass{scrbook}
\usepackage[T1]{fontenc}
\usepackage[german]{babel}
\begin{document}
\tableofcontents

abc\selectlanguage{german} abc \selectlanguage{german}
\end{document}

write three times \german in the auxiliaris files, while it should be easy to remember the actual language, and so not writing anything if not required.

https://chat.stackexchange.com/transcript/message/56242919#56242919

[jbezos]

I was working on a ‘single’ mode for monolingual documents (there are even some parts already written in the current babel), but babel currently works and therefore it was not among my priorities.

[u-fischer]

it is not really about monolingual documents. If there more languages it can happen nevertheless that some code reselects the already active language:

\documentclass{scrbook}
\usepackage[T1]{fontenc}
\usepackage[german,english]{babel}
\begin{document}
\tableofcontents

abc\selectlanguage{german} abc \selectlanguage{german}

abc\selectlanguage{german} abc \selectlanguage{german}

abc\selectlanguage{english} abc 
\end{document}

One could avoid that the language is selected needlessly by using iflang:

\documentclass{scrbook}
\usepackage[T1]{fontenc}
\usepackage[german,english]{babel}
\usepackage{iflang}
\begin{document}
\tableofcontents

abc\IfLanguageName{german}{}{\selectlanguage{german}} abc \IfLanguageName{german}{}{\selectlanguage{german}}

abc\IfLanguageName{german}{}{\selectlanguage{german}} abc \IfLanguageName{german}{}{\selectlanguage{german}}

abc\IfLanguageName{english}{}{\selectlanguage{english}} abc 
\end{document}

But the question is if this is really a user task. Wouldn't it make sense if babel does this?

[jbezos]

Wouldn't it make sense if babel does this?

Indeed, but when considering floats and the like things are not so simple. And this is not the only issue related to a huge quantity of \selectlanguage's because there is an internal macro growing with each selection (\bbl@language@stack). In the following example \IfLanguageName{english}{}{\selectlanguage{english}} is not the same as just \selectlanguage{english}:

\documentclass{article}

\usepackage[english, german]{babel}
\usepackage{iflang}

\begin{document}

\makeatletter

\protected@write\@auxout{}{\string\@gobble{1 \languagename}}%

\selectlanguage{english}

\begin{figure}
\protected@write\@auxout{}{\string\@gobble{F1 \languagename}}%
% \IfLanguageName{english}{}{\selectlanguage{english}}
\selectlanguage{english}
\protected@write\@auxout{}{\string\@gobble{F2 \languagename}}%
\end{figure}

blah

\selectlanguage{english}
\protected@write\@auxout{}{\string\@gobble{2 \languagename}}%

\end{document}

Admittedly, the result with \IfLanguageName perhaps is wrong (a bug?), but anyway it reveals fixing this issue requires a careful analysis.

[u-fischer]

In the following example \IfLanguageName{english}{}{\selectlanguage{english}} is not the same as just \selectlanguage{english}:

well it is not supposed to be same, after all the first should executed the command less often ;-) But which problem do you see? At a first glance it looked ok to me.

[jbezos]

@u-fischer Look at the aux file.

[jbezos]

As the example in https://github.com/latex3/babel/issues/108#issuecomment-733099374 shows, the asynchronous nature of TeX makes impossible to fix this issue in a general way (in the aux file, the lines with \babel@aux can be misplaced if there are floats). I thought there was some hope with luatex, and in fact the callback process_output_buffer can be used to improve things a bit, but just a little bit, so I don’t think it’s a real solution and, anyway, it doesn’t work with pdftex or xetex.

However, in monolingual documents there is a simple solution (≥3.64):

\babeladjust { select.write = omit }

With this setting, no \babel@aux is written to the aux file. There is a minor bug, but currently it should work in most documents.