Fails if 'fr_FR.UTF-8' is not installed

If the fr_FR.UTF-8 is not installed on the system, sgelt fails:

 [...]
 File "/root/.local/share/pipx/venvs/opla-ssg/lib/python3.12/site-packages/sgelt/__init__.py", line 9, in <module>
    locale.setlocale(locale.LC_TIME, 'fr_FR.UTF-8')
  File "/usr/local/lib/python3.12/locale.py", line 615, in setlocale
    return _setlocale(category, locale)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
locale.Error: unsupported locale setting