Footnotes cause pdflatex error about undefined `\localleftbox` with French language

[mitya57]

Describe the bug

Commit 39e4ea2 introduced the use of \localleftbox, which is apparently only available when latex_engine = "lualatex". With the default latex_engine = "pdflatex", it produces the following error:

! Undefined control sequence.
\FNH@prefntext ->\localleftbox 
                               {}\let \FBeverypar@save \FBeverypar@quote \le...
l.377 \begin{footnote}[1]
                         \sphinxAtStartFootnote
? 
! Emergency stop.
\FNH@prefntext ->\localleftbox 
                               {}\let \FBeverypar@save \FBeverypar@quote \le...
l.377 \begin{footnote}[1]
                         \sphinxAtStartFootnote
!  ==> Fatal error occurred, no output PDF file produced!

Making lualatex the default engine for French will work for me, if it cannot be fixed in other ways.

How to Reproduce

A minimal Sphinx project on which it can be reproduced:

$ cat <<EOF > index.rst
[#f1]_

.. [#f1] Test
EOF
$ echo 'language = "fr"' > conf.py
$ python3 -m sphinx -b latex . latex
$ make -C latex

And a minimal TeX file that is enough to trigger the error:

\def\sphinxdocclass{report}
\documentclass[french]{sphinxmanual}
\usepackage{babel}
\usepackage{sphinx}
\usepackage{hyperref}

\begin{document}
\begin{footnote}[1]
Test
\end{footnote}
\end{document}

Environment Information

Platform:              linux; (Linux-6.16.9+deb14-amd64-x86_64-with-glibc2.41)
Python version:        3.13.9 (main, Oct 15 2025, 14:56:22) [GCC 15.2.0])
Python implementation: CPython
Sphinx version:        8.2.3
Docutils version:      0.21.2
Jinja2 version:        3.1.6
Pygments version:      2.19.2


pdfTeX 3.141592653-2.6-1.40.28 (TeX Live 2025/Debian)

Sphinx extensions

Additional context

Downstream bug report: https://bugs.debian.org/1114478.

@jfbu I hope you don’t mind if I mention you directly here.

[jfbu]

Thanks for thorough report.

\localleftbox is a luatex-engine primitive and presumably older versions of babel-frenchb gave it some dummy definition for all engines. My computers currently hold only the most recent TeXLive, ..., ok I checked on an old laptop I have here where I stay now, it has both TL2019 and TL2025. I see that in version 2020/01/20 v3.5g of frenchb.dtx there was a line

\providecommand\localleftbox[1]{}

and in all probability this was removed, as is logical, when the babel-french code was split around 2025-08-15 v3.7e French support from the babel system into two separate files, one for lualatex on one hand and the other one (renamed frenchb3.dtx) for pdflatex and xelatex only on the other hand.

I fill fix this in Sphinx promptly.