LaTeX: use babel for French with lualatex, not polyglossia

Fix sphinx-doc#12410

doc/latex.rst

@@ -159,17 +159,10 @@ Keys that you may want to override include:
 
    .. versionchanged:: 1.7.6
       For French with ``xelatex`` (not ``lualatex``) the default is to use
-      ``babel``, not ``polyglossia``.  To let ``lualatex`` use ``babel``,
-      use this:
+      ``babel``, not ``polyglossia``.
 
-      .. code-block:: python
-
-         latex_elements = {
-             'polyglossia': '',
-             'babel': r'\usepackage{babel}',
-         }
-
-      in file :file:`conf.py`.
+   .. versionchanged:: 7.4.0
+      For French with ``lualatex`` the default is to use ``babel``.
 
 ``'fontpkg'``
    Font package inclusion. The default is::

sphinx/builders/latex/constants.py

@@ -183,6 +183,11 @@
     },
 
     # special settings for latex_engine + language_code
+    ('lualatex', 'fr'): {
+        # use babel instead of polyglossia by default
+        'polyglossia':  '',
+        'babel':        '\\usepackage{babel}',
+    },
     ('xelatex', 'fr'): {
         # use babel instead of polyglossia by default
         'polyglossia':  '',