PDF output file always generated for 'en' language

[madum31]

Hello,

Following the sphinx build directives https://app.readthedocs.org/projects/sphinx-fr/builds/27686267/ to build Sphinx doc in french,
I tried multiple time and ways to compile the sphinx documentation with language set to 'fr':

• no problem using sphinx build command :
  python -m sphinx -T -b html -d _build/doctrees -D language=fr . _build/html
  which outputs sphinx html documentation in french
• when I try to create the PDF output using simplepdf :
  sphinx-build -M simplepdf -D language=fr . _build/pdf
  I always get the PDF file in 'en' language.

Do I missed something elsewhere ?
Or is this locale feature unavailable with sphinx-simplepdf

Thanks for help
best regards,
Martial